Fish (Friendly Interactive Shell)

Fish (Friendly Interactive Shell) is a smart, user-friendly command-line shell featuring autosuggestions, syntax highlighting, and clean scriptable syntax out of the box.

When to Use

Interactive Developer Shell: Out-of-the-box autosuggestions, syntax highlighting, and tab completions without plugins.
Clean Shell Scripting: Intuitive scripting syntax without legacy POSIX quirks (no subshells for math, clear array handling).
Custom Universal Variables: Persisting user variables (set -U) across all shell sessions without editing config files.
High-Speed Shell Startup: Instant startup times (< 20ms) without heavy plugin manager overhead.

Quick Start

# Define and persist a custom function in Fish
function mkcd --description "Create directory and cd into it"
    mkdir -p $argv[1]; and cd $argv[1]
end

# Save function permanently
funcsave mkcd

Core Concepts

Modern Shell Configuration (config.fish)

Configuring environment, completions, and aliases:

# ~/.config/fish/config.fish
if status is-interactive
    # Disable default greeting
    set -g fish_greeting ""

    # Modern path exports
    fish_add_path /opt/homebrew/bin $HOME/.cargo/bin $HOME/.local/bin

    # Environment variables
    set -gx EDITOR nvim
    set -gx KUBECONFIG $HOME/.kube/config

    # Fast shell abbreviations (expanded automatically on Space/Enter)
    abbr -a g git
    abbr -a gco git checkout
    abbr -a k kubectl
    abbr -a d docker
    abbr -a tf terraform

    # Initialize Starship prompt if installed
    if command -q starship
        starship init fish | source
    end
end

Clean Fish Scripting Syntax

Writing readable, modular functions and loops:

# ~/.config/fish/functions/deploy_service.fish
function deploy_service --description "Build and deploy service container"
    set -l service_name $argv[1]

    if test -z "$service_name"
        echo "Error: Service name argument required"
        return 1
    end

    echo "Deploying $service_name to production cluster..."
    docker build -t "myregistry.com/$service_name:latest" .
    and docker push "myregistry.com/$service_name:latest"
    and kubectl rollout restart deployment/$service_name

    if test $status -eq 0
        echo "Deployment completed successfully!"
    else
        echo "Deployment failed with exit code $status"
    end
end

Universal Variables (set -U)

Setting variables across all current and future shell sessions instantly:

# Persist variable globally across all fish sessions without writing to config.fish
set -U FZF_DEFAULT_OPTS "--height 40% --layout=reverse --border"

Common Patterns

Universal Environment Variables and Custom PATH

Problem: Exported environment variables lost when closing shell session in bash/zsh.

Solution: Use Fish universal variables (set -U):

# Set once; persisted across all shell sessions and reboots permanently
set -U EDITOR nvim
set -U fish_user_paths /opt/homebrew/bin $HOME/.cargo/bin $fish_user_paths

Best Practices

Do:

Use fish_add_path to add directories to $PATH idempotently without duplicates.
Use abbreviations (abbr -a) rather than aliases; abbreviations expand in place, maintaining transparent history.
Wrap interactive configurations inside if status is-interactive ... end to keep non-interactive script startup instant.
Store standalone functions in ~/.config/fish/functions/<name>.fish for automatic lazy loading.

Don't:

Write system administration shell scripts in Fish if POSIX /bin/sh or /bin/bash portability is required.
Use legacy bash syntax (e.g. export FOO=bar or $(command)); use set -gx FOO bar and (command).
Overload config.fish with heavy external subshell calls; benchmark startup with fish --profile.

Troubleshooting

References

Fish Shell Documentation