I have been a big fan of the fish shell lately mostly because it delivers what it promises; works out of the box™️.

The obvious downside to fish is that it is non-standard POSIX sh – meaning some (not all!) of the 1-line scripts you find on the Internet may not work.

I use to be a pretty big zsh fan but I hit enough oddities with my setup that I gave up in anger one day. 😤

In practice, it should not that bad since scripts can set a shebang like #! /usr/bin/env bash to make sure they are portable. In reality, I’ve been bit too much by tools that run commands without explicitly setting the shell and relying on my login $SHELL. 🙅‍♂️🤯

Here is a little trick I’ve used to get the best of both.

1. Set your login shell to bash, even though we intend to use fish.

    users.extraUsers.fmzakari = {
        isNormalUser = true;
        shell = pkgs.bash;
        extraGroups = ["wheel" "networkmanager"];
        description = "Farid Zakaria";
    };
   

2. Set your .bash_profile or (.bashrc conditional on interactive shell) to exec into fish.

    bash = {
        enable = true;
        initExtra = ''
        # I have had so much trouble running fish as my login shell
        # instead run bash as my default login shell but just exec into it.
        # Check if the shell is interactive.
        if [[ $- == *i* && -z "$NO_FISH_BASH" ]]; then
            exec ${pkgs.fish}/bin/fish
        fi
        '';
    };
   

3. Create a bash function in fish that starts bash with the secret environment variable NO_FISH_BASH so that we don’t get into an endless loop.

    programs.fish = {
        enable = true;
        functions = {
            # to avoid going into a loop from bash -> fish -> bash
            # set the environment variable which stops that.
            bash = {
            body = ''
                NO_FISH_BASH="1" command bash $argv
            '';
            wraps = "bash";
            };
        };
    };
   

Now when you start your shell, via an interactive session, it will automatically exec into a fish shell 🎉.

AND

Your $SHELL remains bash, which means that any non-interactive use by programs will get the common bash they unfortunately implicitly rely on.

❯ ps -p $fish_pid
    PID TTY          TIME CMD
  71116 pts/2    00:00:00 fish

❯ echo $SHELL
/run/current-system/sw/bin/bash

❯ bash -c "echo $SHELL"
/run/current-system/sw/bin/bash

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