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
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