Fish Shell Configuration Skill

Fish is not POSIX. Every pattern here targets Fish 3.0+ (supports $(), &&, ||). Fish 4.0 (Rust rewrite) has no syntax changes. All generated code must use Fish-native syntax exclusively — never emit Bash constructs (VAR=value, [[ ]], export, heredocs) in Fish contexts.

Instructions

Step 1: Confirm Fish Context

Before writing any shell code, confirm the target is Fish:

• $SHELL contains fish, or
• Target file has .fish extension, or
• Target directory is ~/.config/fish/

If none of these hold, stop — this skill does not apply to Bash, Zsh, or POSIX shells.

Step 2: Choose the Correct File Location

Place configuration in conf.d/ modules with numeric prefixes for ordering — keep config.fish minimal. A monolithic config.fish with hundreds of lines is slow to load, hard to maintain, and impossible to selectively disable.

Directory layout:

~/.config/fish/
├── config.fish              # Minimal — interactive-only init
├── fish_variables           # Auto-managed by Fish (never edit)
├── conf.d/                  # Auto-sourced in alphabetical order
│   ├── 00-path.fish
│   ├── 10-env.fish
│   └── 20-abbreviations.fish
├── functions/               # Autoloaded functions (one per file)
│   ├── fish_prompt.fish
│   └── mkcd.fish
└── completions/             # Custom completions
    └── mycommand.fish

Decision tree:

Step 3: Write Variables

Variable assignment is always set VAR value — never VAR=value (syntax error in Fish) or export VAR=value.

set -l VAR value    # Local — current block only
set -f VAR value    # Function — entire function scope
set -g VAR value    # Global — current session
set -U VAR value    # Universal — persists across sessions (use sparingly)
set -x VAR value    # Export — visible to child processes
set -gx VAR value   # Global + Export (typical for env vars)
set -e VAR          # Erase variable
set -q VAR          # Test if set (silent, for conditionals)

Every Fish variable is a list. Never use colon-separated strings for PATH or similar variables — set PATH "$PATH:/new/path" creates a single malformed element because Fish PATH is a list, not a colon-delimited string.

Step 4: Manage PATH

Use fish_add_path for PATH manipulation — it handles deduplication and persistence automatically. Manual set PATH only for session-scoped overrides.

# CORRECT: fish_add_path handles deduplication and persistence
fish_add_path ~/.local/bin
fish_add_path ~/.cargo/bin
fish_add_path -P ~/go/bin     # -P = session only, no persist

# CORRECT: Direct manipulation when needed (session only)
set -gx PATH ~/custom/bin $PATH

# WRONG: Colon-separated string — Fish PATH is a list
# set PATH "$PATH:/new/path"

Step 5: Write Functions

The autoloaded function filename must match the function name exactly — functions/foo.fish must contain function foo. A mismatch causes "Unknown command" errors.

# ~/.config/fish/functions/mkcd.fish
function mkcd --description "Create directory and cd into it"
    mkdir -p $argv[1]
    and cd $argv[1]
end

Functions with argument parsing:

function backup --description "Create timestamped backup"
    argparse 'd/dest=' 'h/help' -- $argv
    or return

    if set -q _flag_help
        echo "Usage: backup [-d destination] file..."
        return 0
    end

    set -l dest (set -q _flag_dest; and echo $_flag_dest; or echo ".")
    for file in $argv
        set -l ts (date +%Y%m%d_%H%M%S)
        cp $file $dest/(basename $file).$ts.bak
    end
end

Step 6: Choose Between Abbreviations, Functions, and Aliases

Abbreviations are interactive-only — they do not work in scripts. Always wrap them in an interactive guard because they have no effect during non-interactive sourcing:

# Always guard abbreviations
if status is-interactive
    abbr -a g git
    abbr -a ga "git add"
    abbr -a gc "git commit"
    abbr -a gst "git status"
    abbr -a dc "docker compose"
end

Step 7: Write Conditionals and Control Flow

Use the test builtin for conditionals — never [[ ]] (syntax error in Fish) or [ ] (calls external /bin/[, slower than the builtin). Fish has no word splitting, so $var and "$var" behave identically — quote only when you need to prevent list expansion or preserve empty strings.

# Conditionals — use 'test', not [[ ]]
if test -f config.json
    echo "exists"
else if test -d config
    echo "is directory"
end

# Command chaining (both styles work in Fish 3.0+)
mkdir build && cd build && cmake ..
mkdir build; and cd build; and cmake ..

# Loops
for file in *.fish
    echo "Processing $file"
end

# Switch
switch $argv[1]
    case start
        echo "Starting..."
    case stop
        echo "Stopping..."
    case "*"
        echo "Unknown: $argv[1]"
        return 1
end

Step 8: Integrate External Tools

Guard every tool integration with type -q so the config works on machines where the tool is not installed:

# ~/.config/fish/conf.d/30-tools.fish
if type -q starship
    starship init fish | source
end

if type -q direnv
    direnv hook fish | source
end

if type -q fzf
    fzf --fish | source
end

# Homebrew (macOS)
if test -x /opt/homebrew/bin/brew
    eval (/opt/homebrew/bin/brew shellenv)
end

# Nix
if test -e /nix/var/nix/profiles/default/etc/profile.d/nix-daemon.fish
    source /nix/var/nix/profiles/default/etc/profile.d/nix-daemon.fish
end

Step 9: Verify

1. Syntax check — run fish -n <file> (parse without executing)
2. Function name match — verify filename matches function name for every file in functions/
3. Interactive guards — verify status is-interactive guards on abbreviations and key bindings in conf.d/
4. Clean environment test — run fish --no-config then source <file> to confirm isolated correctness

Reference Material

Example: Setting Up a New Fish Config

User says: "Set up my Fish shell config"

1. Confirm Fish context
2. Create modular structure in ~/.config/fish/
3. Write conf.d/00-path.fish, conf.d/10-env.fish, conf.d/20-abbreviations.fish
4. Syntax-check all files

Example: Migrating a Bash Alias File

User says: "Convert my .bash_aliases to Fish"

1. Read .bash_aliases, confirm Fish target
2. Determine which become abbreviations vs functions
3. Write abbreviations to conf.d/, functions to functions/
4. Syntax-check, test in clean shell

Error Handling

Error: "Unknown command" for new function

Cause: Filename does not match function name Solution: Ensure functions/foo.fish contains exactly function foo. Check for typos in both the filename and the function declaration.

Error: PATH changes not persisting across sessions

Cause: Used set -gx PATH (session-only) instead of fish_add_path (writes to universal fish_user_paths) Solution: Use fish_add_path /new/path which persists by default, or use set -U fish_user_paths /path $fish_user_paths explicitly.

Error: Abbreviations not expanding in scripts

Cause: Abbreviations are interactive-only by design Solution: Use a function instead. Move the logic from abbr to a file in functions/.

Error: Variable not visible to child process

Cause: Missing -x (export) flag on set Solution: Use set -gx VAR value to make variable visible to subprocesses. Check with set --show VAR to inspect current scope and export status.

References

• ${CLAUDE_SKILL_DIR}/references/bash-migration.md: Complete Bash-to-Fish syntax translation table
• ${CLAUDE_SKILL_DIR}/references/fish-quick-reference.md: Variable scoping, special variables, and command cheatsheet