Add a straightforward way to determine if inside a nix3 shell

[9p4]

Motivation

I just want to have my Fish prompt tell me if I am in a Nix3 shell.

Context

See: #6677 #3862

Checklist for maintainers

Maintainers: tick if completed or explain if not relevant

• [ ] agreed on idea
• [ ] agreed on implementation strategy
• [ ] tests, as appropriate
  • functional tests - tests/**.sh
  • unit tests - src/*/tests
  • integration tests - tests/nixos/*
• [ ] documentation in the manual
• [ ] documentation in the internal API docs
• [ ] code and comments are self-explanatory
• [ ] commit message explains why the change was made
• [ ] new feature or incompatible change: updated release notes

Priorities

Add :+1: to pull requests you find important.

Fixes #3862 #6677

[Diti]

This pull request has been in standby for a year. Any news?

[9p4]

It is Nix, so you can just apply this patch with an overlay:

https://git.sr.ht/~fd/nix-configs/tree/dea481544e44e1289d88f802f3eaa2cc23fdf63c/item/overlays/default.nix#L30

https://git.sr.ht/~fd/nix-configs/tree/dea481544e44e1289d88f802f3eaa2cc23fdf63c/item/nixos/common/default.nix#L124

Be sure to not name the modified package nix because it will then change the hashes for a bunch of Electron dependencies (and cause Chromium to compile from scratch, not fun).

As I was writing this blog post, I figured out the problem: electron depends on prefetch-npm-deps which in turn depends on nix itself. However, I had patched the Nix package to set an environment variable when running in a nix3 shell, so this broke the chain of dependencies and thus caused prefetch-npm-deps and everything that depended on it to recompile. What caused me to look into this was that I noticed that both jellyfin-media-player and prefetch-npm-deps were being recompiled on update. Going into my flake.nix and removing the overlays section caused Electron's hashes to match, so it was only a few minutes until I figured out that the Nix patches caused the issue. Renaming the Nix package to customnix and setting the nix.package setting to use customnix fixed everything. Commit which fixed the issue. ↩

[amuckstot30]

Personally, to avoid having recompile nix I just make a wrapper:

nix.package =
  pkgs.symlinkJoin
  {
    name = "nix-custom";
    version = pkgs.nix.version;
  
    paths = [
      pkgs.nix
    ];

    buildInputs = [pkgs.makeWrapper];

    postBuild = ''
      wrapProgram $out/bin/nix \
        --run "[ \"\$1\" = \"shell\" ] \
        && export IN_NIX_SHELL=\"impure\" \
        && export name=\"shell\""
    '';
  };

Perhaps it's not the cleanest solution, but it gets the job done.

[stelcodes]

The fact that Lix has now added this common sense feature before Nix has (after a 15 month head start) is absurd.