Gabriella439 / nix-diff

Explain why two Nix derivations differ

nix-diff

This package provides a nix-diff executable which explains why two Nix derivations differ. The most common use cases for this are:

• Understanding why you have a cache miss
• Verifying a Nix change had the intended effect at the derivation level
• Improving your understanding of how Nix works

For example, consider the following Nix derivation to build a bare-bones NixOS system:

let
  nixos = import <nixpkgs/nixos> {
    system = "x86_64-linux";

    configuration = {
      boot.loader.grub.devices = [ "/dev/sda" ];

      fileSystems."/" = {
        device = "/dev/sda";
      };
    };
  };

in
  nixos.system

We can use nix-instantiate to compute the derivation for the above file:

$ nix-instantiate example.nix  # Your exact hash might differ
warning: you did not specify ‘--add-root’; the result might be removed by the garbage collector
/nix/store/6z9nr5pzs4j1v9mld517dmlcz61zy78z-nixos-system-nixos-18.03pre119245.5cfd049a03.drv

Now, let's add a service to our system definition:

let
  nixos = import <nixpkgs/nixos> {
    system = "x86_64-linux";

    configuration = {
      boot.loader.grub.devices = [ "/dev/sda" ];

      fileSystems."/" = {
        device = "/dev/sda";
      };

      services.apache-kafka.enable = true;
    };
  };

in
  nixos.system

... and compute the derivation for the updated system:

$ nix-instantiate example.nix
warning: you did not specify ‘--add-root’; the result might be removed by the garbage collector
/nix/store/k05ibijg0kknvwrgfyb7dxwjrs8qrlbj-nixos-system-nixos-18.03pre119245.5cfd049a03.drv

We can use nix-diff to compare the two computed derivations to determine what changed about our system:

$ nix-diff /nix/store/6z9nr5pzs4j1v9mld517dmlcz61zy78z-nixos-system-nixos-18.03pre119245.5cfd049a03.drv /nix/store/k05ibijg0kknvwrgfyb7dxwjrs8qrlbj-nixos-system-nixos-18.03pre119245.5cfd049a03.drv

... which produces the following output:

It's also possible to pass store paths or links to store paths, for example:

$ nix-build example.nix
$ nix-diff /run/current-system ./result

Testing

Run tests, using cabal test command.

You need to have nix installed in your PC.

Development status

I don't currently plan to add any new features, but I do welcome feature requests. Just open an issue on the issue tracker if you would like to request an improvement.

License (BSD 3-clause)

Copyright (c) 2017 Gabriella Gonzalez
All rights reserved.

Redistribution and use in source and binary forms, with or without modification,
are permitted provided that the following conditions are met:
    * Redistributions of source code must retain the above copyright notice,
      this list of conditions and the following disclaimer.
    * Redistributions in binary form must reproduce the above copyright notice,
      this list of conditions and the following disclaimer in the documentation
      and/or other materials provided with the distribution.
    * Neither the name of Gabriella Gonzalez nor the names of other contributors
      may be used to endorse or promote products derived from this software
      without specific prior written permission.

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