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.systemWe 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.drvNow, 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.drvWe 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 ./resultRun tests, using cabal test command.
You need to have nix installed in your PC.
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.
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 ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.