just – Command Runner

just is a command runner that lets you save and run project-specific commands (called recipes) in a justfile. It’s similar to make but focused purely on running commands, not building software.

Documentation Files

This skill includes the complete official documentation from the just repository:

• README.md – Comprehensive documentation including installation, syntax reference, features, and examples (this is the primary reference)
• GRAMMAR.md – Formal grammar specification for justfile syntax
• CHANGELOG.md – Version history and release notes
• CONTRIBUTING.md – Contribution guidelines
• LICENSE – CC0 1.0 Universal license
• examples/ – Example justfiles for various use cases

Quick Reference

Basic Justfile Structure

# This is a comment
variable := "value"

# Recipe with no dependencies
recipe-name:
    echo "Running recipe"

# Recipe with dependencies
build: clean compile
    echo "Build complete"

# Recipe with parameters
greet name:
    echo "Hello, {{name}}!"

# Recipe with default parameter
serve port="8080":
    python -m http.server {{port}}

Running Recipes

just                    # Run default recipe (first in file)
just recipe-name        # Run specific recipe
just recipe arg1 arg2   # Run recipe with arguments
just --list             # List available recipes
just --show recipe      # Show recipe source
just --dry-run recipe   # Show what would run without executing
just --choose           # Interactive recipe selection (requires fzf)

Key Features

• Recipe Parameters – Pass arguments to recipes
• Variables – Define and use variables with :=
• String Interpolation – Use {{variable}} in recipes
• Dotenv Support – Automatically loads .env files
• Shebang Recipes – Write recipes in Python, Node, Ruby, etc.
• Conditional Logic – Use if expressions and functions
• Dependencies – Recipes can depend on other recipes
• Private Recipes – Prefix with _ to hide from listing
• Documentation – Add doc comments with # comment above recipes
• Cross-Platform – Works on Linux, macOS, Windows, BSD

Common Settings

# Load .env file
set dotenv-load

# Use different shell
set shell := ["bash", "-c"]

# Export all variables as environment variables
set export

# Allow recipes with same name as built-in functions
set allow-duplicate-recipes

# Fail immediately on error
set shell := ["bash", "-uc"]

Built-in Functions

# Path functions
home_dir := home_directory()
current := justfile_directory()
parent := parent_directory(current)

# String functions
upper := uppercase("hello")
lower := lowercase("HELLO")
replaced := replace("hello", "l", "x")
trimmed := trim("  spaces  ")

# Environment
value := env_var("HOME")
value_or_default := env_var_or_default("VAR", "default")

# OS detection
os := os()
arch := arch()

# Conditionals
result := if os() == "linux" { "Linux" } else { "Other" }

Shebang Recipes (Other Languages)

# Python recipe
python-example:
    #!/usr/bin/env python3
    import sys
    print(f"Python version: {sys.version}")

# Node.js recipe
node-example:
    #!/usr/bin/env node
    console.log("Hello from Node!");

# Bash with strict mode
bash-example:
    #!/usr/bin/env bash
    set -euxo pipefail
    echo "Strict bash mode"

Common Use Cases

When the user asks to:

• Create a justfile – Reference README.md for syntax and examples
• Add a recipe – Check README.md for recipe syntax patterns
• Use variables/interpolation – See README.md variable section
• Set up for different OS – Check cross-platform and shell settings
• Write recipes in Python/Node/etc – See shebang recipes section
• Understand grammar – Reference GRAMMAR.md for formal specification
• Check version changes – Reference CHANGELOG.md

Tips

• Start recipe names with _ to make them private (hidden from just --list)
• Use @ at start of line to suppress command echoing
• Use - at start of line to continue on error
• Recipes are run from the justfile’s directory by default
• Use just --fmt to format your justfile
• Shell completion scripts are available for bash, zsh, fish, powershell, and more

Resources

• Homepage: https://just.systems/
• GitHub: https://github.com/casey/just
• Book (latest release docs): https://just.systems/man/en/
• Discord: https://discord.gg/ezYScXR
• Crates.io: https://crates.io/crates/just