Fish Shell: An Interactive Shell That Works Out of the Box

The first time you use Fish, you’ll wonder why other shells don’t even ship with autocomplete by default.

Fish (Friendly Interactive Shell) is a shell designed for interactive use. It comes with syntax highlighting, autosuggestions, and fuzzy search built-in—no need to install Oh-My-Zsh and dozens of plugins like with Zsh.

Installation

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NixOS:

users.defaultUserShell = pkgs.fish;
programs.fish.enable = true;

Or via Home Manager:

programs.fish = {
  enable = true;
  interactiveShellInit = ''
    set -g fish_greeting
  '';
};

Why Switch from Zsh to Fish

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Fish’s philosophy is “the default should be the best.”

Daily Usage

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Autosuggestions

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As you type, Fish shows suggestions in grey based on history and context:

$ git comm
         ↑ grey text: "git commit -m "
         → Press → or Ctrl+F to accept

Tab Completion

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Press Tab to list all options, with fuzzy matching:

$ cd pro<Tab>
projects/  proposals/  prometheus/

Syntax Highlighting

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Valid commands are green, invalid ones red. Existing paths are blue, non-existent ones have red underlines:

$ ech hello      # "ech" in red (command not found)
$ cat /tmp/test  # "/tmp/test" underlined red (file doesn't exist)

Configuration

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Fish config lives in ~/.config/fish/:

~/.config/fish/
├── config.fish      # Main config
├── conf.d/          # Auto-loaded scripts
│   ├── abbr.fish
│   ├── aliases.fish
│   └── path.fish
└── functions/       # Custom functions

My config.fish

#

# Remove greeting
set -g fish_greeting

# Environment variables
set -gx EDITOR emacsclient -c
set -gx VISUAL emacsclient -c
set -gx LANG en_US.UTF-8

# PATH
fish_add_path ~/.cargo/bin
fish_add_path ~/.local/bin
fish_add_path /nix/var/nix/profiles/default/bin

Abbreviations vs Aliases

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Fish recommends abbr over alias because abbreviations expand as you type, so you can see the actual command:

abbr -a g git
abbr -a ga 'git add'
abbr -a gc 'git commit'
abbr -a gp 'git push'
abbr -a gs 'git status'
abbr -a l 'ls -la'
abbr -a .. 'cd ..'
abbr -a ... 'cd ../..'

Effect: Type gs, press space, it auto-expands to git status.

Custom Functions

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# ~/.config/fish/functions/ll.fish
function ll
    ls -lah $argv
end

# ~/.config/fish/functions/mkcd.fish
function mkcd
    mkdir -p $argv[1]
    cd $argv[1]
end

NixOS-specific Tips

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Auto-detect nix-shell

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# config.fish
if test -n "$IN_NIX_SHELL"
    set -g fish_greeting "📦 nix-shell"
end

direnv + nix-direnv

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# home.nix
programs.direnv = {
  enable = true;
  nix-direnv.enable = true;
};

Automatically loads dev environment when entering a directory with .envrc, unloads when leaving.

Bash Incompatibility Notes

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Fish syntax differs from Bash. For scripts, it’s recommended to:

1. Use #!/usr/bin/env bash in script shebangs — keep writing scripts in Bash
2. Use Fish only for interactive use — don’t write scripts in Fish
3. When copy-pasting Bash commands from the web — most simple commands work, complex pipes may need tweaking

Summary

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If you’re tired of configuring Zsh, Fish is the most hassle-free choice. It won’t turn you into a “shell config expert,” but it will make you slightly happier every time you open a terminal.

The NixOS Toolchain series wraps up here for now. Next up might be some specific Nix configuration tips.