How to look up a Nix package's Nix store path from flake inputs

Sometimes God is dead and you need to figure out what the version of a package in your Nix flake's inputs is. With flakes, you can figure this out using nix eval on a flake reference, but what the hecc is a flake reference?

Every URL has a "fragment", or the thing after the # that mainly tells the browser where to scroll to. Nix flakes uses this to allow you to sub-index on flakes by either name or URL. This allows you to address packages like tmux as:

nixpkgs#legacyPackages.x86_64-linux.tmux

Or my site's code as:

github:Xe/site#packages.x86_64-linux.default

Or even some arbitrary git repo:

git+https://tulpa.dev/cadey/printerfacts.git?ref=main#defaultPackage.x86_64-linux

<Mara> The flake reference is what drills down into the flake so that Nix can tell the difference between part of the path to the repository and the path inside the flake. It usually works on the outputs of a flake that you can find out with nix flake show:

$ nix flake show github:Xe/site
github:Xe/site/5a50cacef85c9e3b9695a2749359b27d0530f86a
├───devShell
│   ├───aarch64-linux: development environment 'nix-shell'
│   └───x86_64-linux: development environment 'nix-shell'
├───nixosModules
│   ├───aarch64-linux: NixOS module
│   └───x86_64-linux: NixOS module
└───packages
    ├───aarch64-linux
    │   ├───bin: package 'xesite-2.4.0'
    │   ├───config: package 'xesite-config-2.4.0'
    │   ├───default: package 'xesite-2.4.0'
    │   ├───posts: package 'xesite-posts-2.4.0'
    │   └───static: package 'xesite-static-2.4.0'
    └───x86_64-linux
        ├───bin: package 'xesite-2.4.0'
        ├───config: package 'xesite-config-2.4.0'
        ├───default: package 'xesite-2.4.0'
        ├───posts: package 'xesite-posts-2.4.0'
        └───static: package 'xesite-static-2.4.0'

<Mara> Tracing through things, you can see how you can go from the package name default to get github.com:Xe/site#packages.x86_64-linux.default as the full legal name of the package!

You can use the nix eval --raw command to figure out what the nix store path of those packages either is or would be:

$ nix eval --raw nixpkgs#legacyPackages.x86_64-linux.tmux
/nix/store/3mn362yak70vq137wnryi7whgvq2cmn2-tmux-3.3a

$ nix eval --raw github:Xe/site#packages.x86_64-linux.default
/nix/store/v3lwgh6vk03yw3aq3j398g613ff3gja0-xesite-2.4.0

$ nix eval --raw git+https://tulpa.dev/cadey/printerfacts.git'?'ref=main#defaultPackage.x86_64-linux
/nix/store/7df21bqkrx2csazc2s8ji8chaabbpqhs-printerfacts-0.3.1

However, this operates on the latest version by default. It won't operate on the current version in your Nix flake. If you need to find it out from the current flake, pass the --inputs-from flag with the current directory .:

$ nix eval --inputs-from . --raw nixpkgs#legacyPackages.x86_64-linux.tmux
/nix/store/5sz57bwrlwhv1l8h4c4sq1bl1fhmcwfw-tmux-3.3a

This will use the nixpkgs defined in your current directory's flake.lock file. You can then share that store path with people to share in your tale of woe as to why a service is crashing when you try and update it.

<Mara> You can also pass in an arbitrary version for a flake URL if you specify it in the URL parameters with ref. You can do it like this: github:Xe/site?ref=5a50cacef85c9e3b9695a2749359b27d0530f86a or by specifying the commit hash as an additional path parameter as in github:Xe/site/5a50cacef85c9e3b9695a2749359b27d0530f86a. Here's an example for this xesite commit:

$ nix flake show github:Xe/site/5a50cacef85c9e3b9695a2749359b27d0530f86a
github:Xe/site/5a50cacef85c9e3b9695a2749359b27d0530f86a
├───devShell
│   ├───aarch64-linux: development environment 'nix-shell'
│   └───x86_64-linux: development environment 'nix-shell'
├───nixosModules
│   ├───aarch64-linux: NixOS module
│   └───x86_64-linux: NixOS module
└───packages
    ├───aarch64-linux
    │   ├───bin: package 'xesite-2.4.0'
    │   ├───config: package 'xesite-config-2.4.0'
    │   ├───default: package 'xesite-2.4.0'
    │   ├───posts: package 'xesite-posts-2.4.0'
    │   └───static: package 'xesite-static-2.4.0'
    └───x86_64-linux
        ├───bin: package 'xesite-2.4.0'
        ├───config: package 'xesite-config-2.4.0'
        ├───default: package 'xesite-2.4.0'
        ├───posts: package 'xesite-posts-2.4.0'
        └───static: package 'xesite-static-2.4.0'

<Cadey> I have reached a point where googling for these questions gets me results on my own blog. Of course they didn't help me in the moment. That's why this post exists. Maybe I'll be able to fix my Matrix server returning that a database migration failed so that I can update my homelab machines because that failing makes the whole deploy rollback. I really hate synapse and I regret setting up my own Matrix homeserver.