Fish

fish, the Friendly Interactive Shell, is a command shell designed around user-friendliness.

Installation

NixOS System Installation

To install fish for a user on a regular nixos system:

❄︎ /etc/nixos/configuration.nix

  programs.fish.enable = true;
  users.extraUsers.myuser = {
    ...
    shell = pkgs.fish;
  };

Replace myuser with the appropriate username.

⚠︎

Warning: As noted in the fish documentation, using fish as your *login* shell (via /etc/passwd) may cause issues, particularly for the root user, because fish is not POSIX compliant. While using fish as the default shell for regular users is generaly safe, caution is still advised. See the Setting fish as default shell section for recommendations and mitigations.

Home Manager

For a user-specific installation managed by Home Manager, use the following configuration:

❄︎ home.nix

home-manager.users.myuser = {
  programs.fish.enable = true;
};

Replace myuser with the appropriate username.

You can enable the fish shell and manage fish configuration and plugins with Home Manager, but to enable vendor fish completions provided by Nixpkgs you will also want to enable the fish shell:

❄︎ /etc/nixos/configuration.nix

  programs.fish.enable = true;

Configuration

Available fish plugins packaged in Nixpkgs can be found via the fishPlugins package set.

NixOS System Configuration

To enable fish plugins system-wide, add your preferred plugins to `environment.systemPackages`:

❄︎ /etc/nixos/configuration.nix

  programs.fish = {
    enable = true;
    interactiveShellInit = ''
      set fish_greeting # Disable greeting
    '';
  };

  environment.systemPackages = with pkgs; [
    ...
    fishPlugins.done
    fishPlugins.fzf-fish
    fishPlugins.forgit
    fishPlugins.hydro
    fzf
    fishPlugins.grc
    grc
    # Use 3rd-party fish plugins manually packaged.
    pkgs.callPackage ../fish-colored-man.nix {buildFishPlugin = pkgs.fishPlugins.buildFishPlugin; } )
  ];

Example of a file containing the definition of a fish plugin.

❄︎ /etc/nixos/fish-colored-man.nix

{
  lib,
  buildFishPlugin,
  fetchFromGitHub,
}:
buildFishPlugin {
  pname = "fish-colored-man";
  version = "0-unstable-20240416";
  src = fetchFromGitHub {
    owner = "decors";
    repo = "fish-colored-man";
    rev = "1ad8fff696d48c8bf173aa98f9dff39d7916de0e";
    hash = "sha256-uoZ4eSFbZlsRfISIkJQp24qPUNqxeD0JbRb/gVdRYlA=";
  };
}

For a full list of fish module options, refer to programs.fish.

Home Manager

An example configuration in Home Manager for adding plugins and changing options could look like this:

❄︎ home.nix

home-manager.users.myuser = {
  programs.fish = {
    enable = true;
    interactiveShellInit = ''
      set fish_greeting # Disable greeting
    '';
    plugins = [
      # Enable a plugin (here grc for colorized command output) from nixpkgs
      { name = "grc"; src = pkgs.fishPlugins.grc.src; }
      # Manually packaging and enable a plugin
      {
        name = "z";
        src = pkgs.fetchFromGitHub {
          owner = "jethrokuan";
          repo = "z";
          rev = "e0e1b9dfdba362f8ab1ae8c1afc7ccf62b89f7eb";
          sha256 = "0dbnir6jbwjpjalz14snzd3cgdysgcs3raznsijd6savad3qhijc";
        };
      }
    ];
  };
};

For the full list of available home-manager options for fish, refer to the module source.

Tips and tricks

Setting fish as default shell

Using fish as the the login shell can cause compatibility issues. For example, certain recovery environments such as systemd's emergency mode to be completely broken when fish was set as the login shell. This limitation is noted on the Gentoo wiki. There they present an alternative, keeping bash as the system shell but having it exec fish when run interactively.

Here is one solution, which launches fish unless the parent process is already fish:

❄︎ /etc/nixos/configuration.nix

programs.bash = {
  interactiveShellInit = ''
    if [[ $(${pkgs.procps}/bin/ps --no-header --pid=$PPID --format=comm) != "fish" && -z ''${BASH_EXECUTION_STRING} ]]
    then
      shopt -q login_shell && LOGIN_OPTION='--login' || LOGIN_OPTION=""
      exec ${pkgs.fish}/bin/fish $LOGIN_OPTION
    fi
  '';
};

If you still want to set fish as the login shell, see Command Shell#Changing the default shell.

Running fish interactively with zsh as system shell on darwin

Zsh users on darwin will need to use a modified version of the above snippet. As written, it presents two incompatibilities. First, being BSD-derived, MacOS's ps command accepts different options. Second, this is a script intended for bash, not zsh. MacOS uses zsh as its default shell.

programs.zsh = {
  initExtra = ''
    if [[ $(ps -o command= -p "$PPID" | awk '{print $1}') != 'fish' ]]
    then
        exec fish -l
    fi
  ''
};

Show that you are in a nix-shell

Add this to the fish_prompt function (usually placed in ~/.config/fish/functions/fish_prompt.fish):

set -l nix_shell_info (
  if test -n "$IN_NIX_SHELL"
    echo -n "<nix-shell> "
  end
)

and $nix_shell_info to the echo in that function, e.g.:

echo -n -s "$nix_shell_info ~>"

Now your prompt looks like this:

• outside: ~>
• inside: <nix-shell> ~>

You can directly start nix-shell in fish with nix-shell --run fish.

Environments

Here are some examples of helper functions that put you in a nix-shell with the given packages installed.

You can either put these in programs.fish.functions with home-manager or in ~/.config/fish/functions/fish_prompt.fish without.

haskellEnv

function haskellEnv
  nix-shell -p "haskellPackages.ghcWithPackages (pkgs: with pkgs; [ $argv ])"
end

# Invocation: haskellEnv package1 packages2 .. packageN

pythonEnv

function pythonEnv --description 'start a nix-shell with the given python packages' --argument pythonVersion
  if set -q argv[2]
    set argv $argv[2..-1]
  end
 
  for el in $argv
    set ppkgs $ppkgs "python"$pythonVersion"Packages.$el"
  end
 
  nix-shell -p $ppkgs
end

# Invocation: pythonEnv 3 package1 package2 .. packageN
# or:         pythonEnv 2 ..

See also

• Command Shell