Unveiling Package Dependencies with 'nix why-depends' (with examples)

The command nix why-depends is a powerful tool within the Nix ecosystem that allows users to investigate the dependency relationships between packages. Whether you’re a developer seeking to understand why a particular package is included in your NixOS system or a maintainer aiming to optimize package builds, nix why-depends offers valuable insights. This tool shines in providing detailed explanations of both runtime and build-time dependencies, thereby assisting in debugging, optimization, and better comprehension of the Nix package relationships.

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

Code:

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

Motivation:

Running a NixOS system means relying on various packages from the Nix store, which is a repository of pre-built binaries and source code managed by Nix. At times, users may find themselves questioning why a particular binary or package is present in their system’s store paths. This can arise when doing system clean-ups, trying to minimize the system footprint, or understanding implicit dependencies. By using nix why-depends, users can pinpoint the exact reason a specific package or store path is part of their system, providing clarity and avoiding unnecessary clutter.

Explanation:

• /run/current-system: This argument indicates the reference point from which the dependency search starts. It represents the current generation of the NixOS system, effectively acting as the list of all packages and configurations presently active on the system.
• /nix/store/...: This path represents the specific store item whose dependency path is being investigated. By stating the store path, the command checks all dependencies that lead to this particular package.

Example Output:

/nix/store/xyz-current-system: … -> /nix/store/abc-package
/nix/store/abc-package: …  -> /nix/store/def-library
/nix/store/def-library: …  -> /nix/store/...

Here, you see the chain of dependencies leading to the inclusion of the store path in question. Each arrow represents a direct dependency from the start-point to the target path.

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

Code:

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

Motivation:

For package maintainers or developers working with the Nix Packages collection (nixpkgs), understanding the build-time dependencies of packages is crucial. This knowledge is essential when optimizing builds, reducing build times, or debugging build issues. Using nix why-depends, one can discern why a specific package requires another during its build process, ultimately streamlining the dependency tree and shaving off unnecessary dependencies.

Explanation:

• --derivation: This option specifies that the command should analyze derivations, which are the low-level build instructions in Nix. This focus on derivations helps uncover the build-time dependency paths.
• nixpkgs#dependent: This specifies the package whose dependencies you want to explore. The nixpkgs# prefix indicates that the package resides within the Nix Packages collection.
• nixpkgs#dependency: This represents the package suspected to be a dependency. The command investigates whether and why this package is required at build time for the dependent package.

Example Output:

/nix/store/ghi-derivation leads to nixpkgs#build-dependency via:
  /nix/store/jkl-used-in-build
  /nix/store/mno-triggered

In this output, you see the path of derivations that confirm why nixpkgs#dependent depends on nixpkgs#dependency. The intermediate store paths show the chain of build steps leading to the dependency inclusion.

Conclusion:

The nix why-depends command is an indispensable tool for unraveling the intricate web of package dependencies in Nix and NixOS systems. Whether it is for maintenance, debugging, or optimization purposes, this command provides clarity on both runtime and build-time dependencies. By using the examples illustrated above, users can unlock deeper insights into their system or development environment, paving the way for a more streamlined and efficient package management experience.