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.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
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