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Allow specifying the fish config directory with FISH_CONFIG_DIR by OleStrohm · Pull Request #13074 · fish-shell/fish-shell · GitHub
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Allow specifying the fish config directory with FISH_CONFIG_DIR - #13074

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Allow specifying the fish config directory with FISH_CONFIG_DIR#13074
OleStrohm wants to merge 1 commit into
fish-shell:masterfrom
OleStrohm:push-llluznkmssrp

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@OleStrohm

@OleStrohm OleStrohm commented Oct 29, 2026 •
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This adds an environment variable that allows the user to specify the fish config directory.
This is primarily aimed at usage with nix, as it will allow making a fully hermetic fish installation that includes the configuration inside it. See example flake.nix:

Details The config would be in the same directory as the flake.nix (Note: This assumes that this commit is merged and updated in nixpkgs) 
{
  description = "A fish installation wrapped with the fish config";
  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs?ref=nixos-unstable";
  };

  outputs = { nixpkgs, ... }:
    let
      pkgs = (import nixpkgs) {
        system = "x86_64-linux";
      };

      fish = pkgs.stdenv.mkDerivation {
        pname = "fish";
        version = "wrapped";

        buildInputs = [ pkgs.makeWrapper ];
        dontUnpack = true;

        installPhase = ''
          mkdir -p $out/
          cp -r ${pkgs.fish}/* $out/
          chmod -R +rw $out/
          rm $out/bin/fish
          makeWrapper ${pkgs.fish}/bin/fish $out/bin/fish --set-default FISH_CONFIG_DIR ${./.}
        '';

        meta = {
          description = "Custom wrapped fish shell";
          mainProgram = "fish";
        };
        passthru = {
          shellPath = "/bin/fish";
        };
      };
  in
  {
    packages.x86_64-linux.default = fish;
  };
}

But I believe that this will also be useful for people wanting to try out someone else's configs more easily (just download it and set CONFIG_FISH_DIR, without affecting your actual installed config in any way)

TODOs:

  • Changes to fish usage are reflected in user documentation/manpages.
  • Tests have been added
  • User-visible changes noted in CHANGELOG.rst

Not sure how to add a test for this to be perfectly honest, so guidance for that would be very helpful!

@OleStrohm

OleStrohm commented Oct 29, 2026

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Looks like the failing test is also failing on latest master

agammann
agammann reviewed Nov 3, 2026
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[P2] Resolve a relative FISH_CONFIG_DIR once against the startup working directory. The new override stores ./my-fish verbatim in the static config path. Startup can source ./my-fish/config.fish, but after cd /tmp, __fish_config_dir and the function/completion paths built from it now point at /tmp/my-fish instead. That silently changes which config files are read and can autoload a different function from the new directory. The documented override does not require an absolute path, so normalize relative values before caching them and add a cd regression test.

@zanchey

zanchey commented Nov 3, 2026

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We've previously decided against this (see #3030), although the idea of a "hermetic portable fish" is interesting. I'm inclined not to merge this because of the additional complexity in interface though.

@OleStrohm

OleStrohm commented Nov 3, 2026

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@zanchey I should've looked to see if there had been prior art, apologies! I can understand the desire not to add complexity to the interface, but the original decision made in #3030 was based on someone wanting to place their fish config in a different directory. That can be handled, as people mention, by either deciding not to, using a symlink, or changing XDG_CONFIG_DIR.
However the use-case I'm proposing is not covered by any of those options. If I change XDG_CONFIG_DIR, then it will impact other software running inside the fish (Just because someone is running a hermetic fish Nix flake does not mean their entire system is Nix-based, so it would mess up their actual XDG_CONFIG_DIR setting. For example I intend to run this on my Ubuntu machine at work in addition to using it in my Nix machine at home).
Using a symlink from ~/.config/fish is not possible as that's not possible in Nix.
And of course I can just not do this, but that's no fun!

I would love to see this supported upstream, as I don't want to rebase this patch every time I want to upgrade fish, but I will if it's not wanted by the project, but hopefully you will reconsider!
Is there any adjustment that can be made that would lower the complexity perhaps?

@agammann I will make the change you suggest and try to figure out how to make a test for this ☺️

@OleStrohm
OleStrohm force-pushed the push-llluznkmssrp branch 2 times, most recently from ea90996 to a3d637c Compare November 3, 2026 23:22
@OleStrohm

OleStrohm commented Nov 3, 2026

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I've made the path absolute and added a test that verifies the behaviour of this feature in general 🎉

@OleStrohm
OleStrohm force-pushed the push-llluznkmssrp branch 2 times, most recently from c2d6684 to 0b71585 Compare November 3, 2026 23:29

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@OleStrohm @zanchey @agammann