Just Command Runner Skill

Just Command Runner Skill

Comprehensive assistance with Just, a handy command runner for saving and running project-specific commands. Just uses a justfile with make-inspired syntax but designed as a general-purpose task executor rather than a build system.

When to Use This Skill

This skill should be triggered when:

• Creating or managing justfile recipes
• Setting up project task automation
• Replacing Make or shell scripts with Just
• Working with cross-platform command execution
• Organizing project-specific commands
• Implementing task dependencies and workflows

Quick Reference

Common Patterns

Basic Recipe:

recipe-name:
  echo 'This is a recipe!'

# Silent recipe (@ suppresses echo)
silent-recipe:
  @echo 'This runs quietly'

Recipe with Parameters:

build target='all':
  @echo 'Building {{target}}…'

# Variadic parameters
test *files:
  pytest {{files}}

Recipe with Dependencies:

build:
  cc main.c -o main

test: build
  ./​test --all

Shebang Recipes (multi-language support):

python:
  #!/​usr/​bin/​env python3
  print('Hello from Python!')

node:
  #!/​usr/​bin/​env node
  console.log('Hello from Node!');

Variables and Expressions:

compiler := 'gcc'
flags := '-Wall -O2'

build:
  {{compiler}} {{flags}} main.c -o main

Common Commands

• just — Run default recipe
• just RECIPE — Run specific recipe
• just --list — List all recipes
• just --show RECIPE — Display recipe definition
• just --choose — Interactive recipe selector (requires fzf)
• just --fmt — Format justfile
• just --dump — Output formatted justfile

Example Code Patterns

Example 1 (basic justfile):

# Default recipe runs when you type 'just'
[default]
build:
  cargo build --release

# Recipe with confirmation prompt
[confirm]
deploy:
  kubectl apply -f deployment.yaml

# Platform-specific recipes
[linux]
install:
  apt-get install my-package

[macos]
install:
  brew install my-package

Example 2 (with variables and .env):

# Load .env file
set dotenv-load

# Variables
app_name := env('APP_NAME', 'myapp')
version := `git describe --tags`

# Recipe using variables
build:
  docker build -t {{app_name}}:{{version}} .

# Conditional logic
deploy environment='staging':
  #!/​usr/​bin/​env bash
  if [ "{{environment}}" = "production" ]; then
    echo "Deploying to production..."
  else
    echo "Deploying to staging..."
  fi

Example 3 (modules and imports):

# Import external justfile
import 'tasks/​docker.just'

# Use module system
mod database 'tasks/​db.just'

# Call module recipe
migrate: database::migrate

Reference Files

This skill includes comprehensive documentation in references/:

• just-full-reference.md - Complete Just documentation covering all features
• index.md - Quick reference guide

Use view to read specific reference files when detailed information is needed.

Working with This Skill

For Beginners

Start with the basic recipe patterns above. A simple justfile with a few recipes is all you need to get started. Just provides helpful error messages that point to specific issues in your justfile.

For Specific Features

• Task Automation: Create recipes for common project tasks
• Cross-Platform: Use platform-specific attributes [linux], [macos], [windows]
• Dependencies: Chain recipes with dependency syntax recipe: dep1 dep2
• Multi-Language: Use shebang recipes for Python, Node, Ruby, etc.
• Environment: Load .env files with set dotenv-load

For Code Examples

The quick reference section contains common patterns. For complete syntax and advanced features, consult the reference files.

Key Features

Core Strengths

• Simpler than Make: No .PHONY declarations needed
• Cross-platform: Linux, macOS, Windows, BSD support
• Clear errors: Specific error messages with source context
• Static validation: Unknown recipes and circular dependencies caught early
• Auto .env loading: Environment variables from .env files
• Subdirectory execution: Run from any subdirectory containing justfile

Advanced Capabilities

• 50+ built-in functions: Path manipulation, string operations, hashing, system info
• Multi-language support: Python, Node.js, Perl, Ruby, Nushell, and more
• Module system: Organize complex projects with submodules
• Imports: Include external justfiles
• Interactive chooser: Select recipes with --choose (uses fzf)
• Recipe attributes: [default], [private], [confirm], platform-specific

Installation

Cargo (Rust):

cargo install just

Homebrew (macOS/​Linux):

brew install just

Pre-built binaries:

curl --proto '=https' --tlsv1.2 -sSf https://just.systems/​install.sh | bash -s -- --to /​usr/​local/​bin

Package managers: Available via apt, pacman, dnf, chocolatey, scoop, and 15+ others

Configuration

Shell Settings:

# Set default shell (Unix-like)
set shell := ["bash", "-uc"]

# Windows PowerShell
set windows-shell := ["powershell.exe", "-Command"]

Common Settings:

set dotenv-load          # Auto-load .env files
set export               # Export all variables as environment vars
set positional-arguments # Pass args as positional ($1, $2, etc.)

Resources

references/

Organized documentation extracted from official Just documentation and README:

• Complete syntax reference
• All built-in functions
• Recipe attributes and features
• Examples and patterns

scripts/

Add helper scripts here for common Just automation tasks.

assets/

Add example justfiles or templates here.

Editor Support

Syntax highlighting available for:

• Vim/​Neovim (built-in since version 9.1.1042/​0.11)
• VS Code (community extension)
• Emacs (just-mode)
• JetBrains IDEs
• Helix (built-in since 23.05)
• Zed, Sublime Text, Kakoune, Micro

Notes

• Just version 1.0+ guarantees backwards compatibility
• Unstable features require --unstable flag or set unstable
• Official documentation: https://just.systems/man/en/
• Examples: https://github.com/casey/just/tree/master/examples

Updating

To refresh this skill with updated documentation:

1. Visit https://github.com/casey/just
2. Check for new features in the README and changelog
3. Update reference files with new content