Just Command Runner Skill

just is a command runner (not a build system) that saves and runs project-specific commands in a file called justfile. It uses make-inspired syntax but is simpler, more portable, and avoids make's idiosyncrasies like .PHONY, tab sensitivity issues, and implicit rules.

When to Use Just vs Make

Scenario	Tool
Project task automation (build, test, deploy, lint)	just
Actual file-based build dependencies (compile .c to .o)	make
Cross-platform command runner	just
Legacy projects already deep in make	make (or migrate)

Core Concepts

Recipe = Target

Recipes are the core unit. Each recipe is a named set of commands:

recipe-name:
    command1
    command2

Indentation MUST be consistent (spaces or tabs, but pick one — set ignore-comments helps)
Each line runs in a separate shell by default (use set shell or shebang recipes for multi-line scripts)
No .PHONY needed — just doesn't track file timestamps

Variables

# Assignment
version := "1.0.0"

# Backtick evaluation (runs command, captures stdout)
git_hash := `git rev-parse --short HEAD`

# Environment variable access
home := env('HOME')

# Export to recipe commands
export DATABASE_URL := "postgres://localhost/mydb"

Settings Block

Place settings at the top of the justfile:

set dotenv-load           # Load .env file
set positional-arguments  # Pass recipe args as $1, $2, etc.
set shell := ["bash", "-euco", "pipefail"]  # Fail-fast shell
set export                # Export all variables as env vars
set quiet                 # Suppress command echoing (like @ in make)

Recipe Arguments

# Required argument
deploy target:
    echo "Deploying to {{target}}"

# Default value
greet name="World":
    echo "Hello {{name}}"

# Variadic (one or more)
test +targets:
    go test {{targets}}

# Variadic (zero or more)
lint *flags:
    eslint {{flags}} src/

Dependencies

# Run 'build' before 'test'
test: build
    cargo test

# Pass arguments to dependencies
push: (deploy "production")

# Multiple dependencies
all: clean build test lint

Conditional Logic

# Ternary-style conditional
rust_target := if os() == "macos" { "aarch64-apple-darwin" } else { "x86_64-unknown-linux-gnu" }

# In recipes
check:
    if [ -f .env ]; then echo "Found .env"; fi

Platform-Specific Recipes

[linux]
install:
    sudo apt install ripgrep

[macos]
install:
    brew install ripgrep

[windows]
install:
    choco install ripgrep

Recipe Attributes

[private]           # Hidden from --list
[no-cd]             # Don't cd to justfile directory
[confirm]           # Ask confirmation before running
[confirm("Deploy to production?")]
[no-exit-message]   # Suppress error message on failure
[group("deploy")]   # Group in --list output
[doc("Run the full test suite")]  # Custom doc string

Shebang Recipes (Multi-line Scripts)

When a recipe needs to run as a single script rather than line-by-line:

process-data:
    #!/usr/bin/env python3
    import json
    with open("data.json") as f:
        data = json.load(f)
    print(f"Found {len(data)} records")

Self-Documenting Help

# The default recipe — runs when you type `just` with no args
[private]
default:
    @just --list --unsorted

Comments above recipes become descriptions in just --list:

# Initialize Terraform with backend config
init:
    terraform init

Imports and Modules

# Import another justfile
import 'ci.just'

# Module (namespaced)
mod deploy 'deploy.just'
# Usage: just deploy::production

References

references/justfile-patterns.md — Common justfile patterns by project type
references/make-migration.md — Makefile-to-justfile migration guide

Useful Functions

Function	Purpose
`os()`	Current OS (`linux`, `macos`, `windows`)
`arch()`	CPU architecture (`x86_64`, `aarch64`)
`env('KEY')`	Get environment variable (aborts if missing)
`env('KEY', 'default')`	Get env var with fallback
`invocation_directory()`	Directory where `just` was called from
`justfile_directory()`	Directory containing the justfile
`join(a, b)`	Join path components
`parent_directory(path)`	Parent of path
`file_name(path)`	Filename component
`without_extension(path)`	Remove file extension
`uppercase(s)`	Uppercase string
`lowercase(s)`	Lowercase string
`replace(s, from, to)`	String replacement
`trim(s)`	Trim whitespace
`quote(s)`	Shell-quote a string
`sha256_file(path)`	SHA-256 hash of file
`shell(cmd, args...)`	Execute command, capture output

Installation

# macOS
brew install just

# Cargo
cargo install just

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

# Shell completions
just --completions zsh > ~/.zsh/completions/_just
just --completions bash > /etc/bash_completion.d/just

justfile-skill