reg-flake-input

One of the selling points of nix flakes is finally saying goodbye to channels, and all their issues.

And indeed, once you have converted things like a NixOS system, or just a user profile (on NixOS, Darwin, or whatever) to a flake, then the dependencies are nicely pinned. But there remain a few operations that default to the channel-provided nixpkgs, such as:

• nix shell nixpkgs#<package> (along with nix run etc)
• nix-shell uses (eg where shebang support is needed, which is not yet present in the new flake-aware commands)
• any shell.nix files lying around that still import <nixpkgs>

The tool here is intended to mop up these corner cases. The idea is that if you have eg your user profile specified in a flake, then you just run reg-flake-input, and this pins all these corner cases to the nixpkgs specified in that flake.lock file.

In this way, you can upgrade all these instances at once, simply by running reg-flake-input again whenever you update that flake.lock file, and hopefully never have to think about channels ever again.

The mechanism is twofold:

• Create a user nix registry entry for nixpkgs (or whatever) matching the flake.lock definition. This covers use cases like nix shell nixpkgs#<package>, nix run nixpkgs#<package> etc.

• Ensure this version of nixpkgs is in the nix store, and add a per-user gcroot to preserve it across garbage collection, and write out a ~/.nix-path file that updates NIX_PATH with the appropriate nixpkgs= entry.

  This means that if you source ~/.nix-env from your ~/.zshrc, you are also covered for the nix-shell and import <nixpkgs> cases above. (Note: currently this is limited to zsh.)

This explanation was all in the mainline case (nixpkgs specifically, with a local flake.lock file, using the user nix registry), but you can override these defaults with command-line arguments:

• --lock-file specifies a different flake.lock file
• --entry-name specifies a different package than nixpkgs
• --registry-file specifies a different file than the user nix registry file

Installing

You can just run this flake on-demand, eg:

nix run github:simonchatts/reg-flake-inputs# -- --help

displays the help text.

You can also include the flake in a persistent profile. In the profile flake (eg user or system profile) include github:simonchatts/reg-flake-input as a flake input, add its overlay output to the profile overlays, and then use the reg-flake-input package in the profile.