Just Command Runner

Overview

Expert guidance for Just, a command runner with syntax inspired by make. Use this skill for creating justfiles, writing recipes, configuring settings, and implementing task automation workflows.

Key capabilities:

• Create and organize justfiles with proper structure
• Write recipes with attributes, dependencies, and parameters
• Configure settings for shell, modules, and imports
• Use built-in constants for terminal formatting
• Implement check/write patterns for code quality tools

Quick Reference

Essential Settings

set allow-duplicate-recipes       # Allow recipes to override imported ones
set allow-duplicate-variables     # Allow variables to override imported ones
set shell := ["bash", "-euo", "pipefail", "-c"]  # Strict bash with error handling
set unstable                      # Enable unstable features (user-defined functions, eager keyword)
set dotenv-load                   # Auto-load .env file
set positional-arguments          # Pass recipe args as $1, $2, etc.
set lazy                          # Defer evaluation of unused variables (v1.48.0+)
set no-cd                         # Don't change to justfile directory for any recipe (v1.51.0+)

Common Attributes

Attribute	Purpose
[arg("p", long, ...)]	Configure parameter as --flag option (v1.46)
[arg("p", pattern="…")]	Constrain parameter to match regex pattern
[confirm("prompt")]	Require user confirmation (expressions OK as of v1.49)
[doc("text")]	Override recipe documentation
[env("NAME", "VALUE")]	Set env var for this recipe only (v1.47+, expr v1.51)
[group("name")]	Group recipes in just --list output
[linux] / [macos] …	Restrict to OS; also [freebsd]/[netbsd]/[dragonflybsd] (v1.47+)
[no-cd]	Don't change to justfile directory
[parallel]	Run direct dependencies concurrently
[positional-arguments]	Enable positional args for this recipe only
[private]	Hide from just --list (same as _ prefix)
[script]	Execute recipe as single script block
[script("interpreter")]	Use specific interpreter (bash, python, etc.)
[working-directory: "…"]	Run from given path (expressions OK as of v1.51)

Recipe Argument Flags (v1.46.0+)

The [arg()] attribute configures parameters as CLI-style options:

# Long option (--target)
[arg("target", long)]
build target:
    cargo build --target {{ target }}

# Short option (-v)
[arg("verbose", short="v")]
run verbose="false":
    echo "Verbose: {{ verbose }}"

# Combined long + short
[arg("output", long, short="o")]
compile output:
    gcc main.c -o {{ output }}

# Flag without value (presence sets to "true")
[arg("release", long, value="true")]
build release="false":
    cargo build {{ if release == "true" { "--release" } else { "" } }}

# Help string (shown in `just --usage`)
[arg("target", long, help="Build target architecture")]
build target:
    cargo build --target {{ target }}

Usage examples:

just build --target x86_64
just build --target=x86_64
just compile -o main
just build --release
just --usage build    # Show recipe argument help

Multiple attributes can be combined:

[no-cd, private]
[group("checks")]
recipe:
    echo "hello"

Built-in Constants

Terminal formatting constants are globally available (no definition needed):

Constant	Description
CYAN, GREEN, RED, YELLOW, BLUE, MAGENTA	Text colors
BOLD, ITALIC, UNDERLINE, STRIKETHROUGH	Text styles
NORMAL	Reset formatting
BG_*	Background colors (BG_RED, BG_GREEN, etc.)
HEX, HEXLOWER	Hexadecimal digits

Usage:

@status:
    echo -e '{{ GREEN }}Success!{{ NORMAL }}'
    echo -e '{{ BOLD + CYAN }}Building...{{ NORMAL }}'

Key Functions

# Require executable exists (fails recipe if not found)
jq := require("jq")

# Get environment variable with default
log_level := env("LOG_LEVEL", "info")

# Get justfile directory path
root := justfile_dir()

# Module location (v1.49.0–1.50.0; useful inside `mod` files)
mod_path := module_path()            # Full submodule path, e.g. "foo::bar"
mod_file := module_file()            # Absolute path to module's justfile
mod_dir := module_directory()        # Directory containing the module justfile

# Runtime directory (v1.49.0; typically $XDG_RUNTIME_DIR, falls back to tempdir)
rt := runtime_directory()

User-Defined Functions (v1.49.0+)

Define reusable named expressions with name(args) := expression. Requires set unstable. Functions can reference module-level assignments.

set unstable

base := "foo"
join(extension) := base + "." + extension

# Use f-strings for interpolation
hello(name) := f"Hello, {{ name }}!"

create:
    touch {{ join("c") }}
    touch {{ join("html") }}
    echo '{{ hello("World") }}'

Use these to dedupe expression logic that would otherwise repeat across recipes; prefer them over backtick-evaluated variables when the value depends on input.

Recipe Patterns

When designing recipes that use status reporting, check/write semantics, or alias conventions, see references/patterns.md.

Inline Scripts

When writing recipes that need shell scripts (script attribute or shebang style), see references/inline-scripts.md.

Modules & Imports

Import Pattern

Include recipes from another file:

import "./just/settings.just"
import "./just/base.just"
import? "./local.just"    # Optional (no error if missing)

Module Pattern

Load submodule (requires set unstable):

mod foo                   # Loads foo.just or foo/justfile
mod bar "path/to/bar"     # Custom path
mod? optional             # Optional module

# Call module recipes
just foo::build

Devkit Import Pattern

For projects using @sablier/devkit:

import "./node_modules/@sablier/devkit/just/base.just"
import "./node_modules/@sablier/devkit/just/npm.just"

Section Organization

Standard section header format:

# ---------------------------------------------------------------------------- #
#                                 DEPENDENCIES                                 #
# ---------------------------------------------------------------------------- #

Common sections (in order):

1. DEPENDENCIES - Required tools with URLs
2. CONSTANTS - Glob patterns, environment vars
3. RECIPES / COMMANDS - Main entry points
4. CHECKS - Code quality recipes
5. UTILITIES / INTERNAL HELPERS - Private helpers

Default Recipe

Always define a default recipe:

# Show available commands
default:
    @just --list

Dependencies Declaration

Document required tools at the top:

# ---------------------------------------------------------------------------- #
#                                 DEPENDENCIES                                 #
# ---------------------------------------------------------------------------- #

# Bun: https://bun.sh
bun := require("bun")

# Ni: https://github.com/antfu-collective/ni
na := require("na")
ni := require("ni")
nlx := require("nlx")

# Usage: invoke directly in recipes (not with interpolation)
build:
    bun next build

Note: require() validates the tool exists at recipe evaluation time. Use the variable name directly (e.g., bun), not with interpolation ({{ bun }}).

Context7 Fallback

For Just features not covered in this skill (new attributes, advanced functions, edge cases), fetch the latest documentation:

Use context7 MCP with library ID `/websites/just_systems_man_en` to get up-to-date Just documentation.

Example topics to search:

• modules import mod - Module system details
• settings - All available settings
• attributes - Recipe attributes
• functions - Built-in functions
• script recipes - Script block syntax

Additional Resources

Reference Files

For detailed patterns and comprehensive coverage, consult:

• references/settings.md - Settings configuration and module system
• references/recipes.md - Recipe attributes, parameters, dependencies, and prefixes
• references/syntax.md - Constants, functions, variables, and CLI options
• references/patterns.md - Established conventions, section organization, helper patterns

Example Templates

Working justfile templates in examples/:

• devkit.just - Minimal template importing @sablier/devkit
• standalone.just - Full standalone template with all patterns

External Documentation

• Official Manual: https://just.systems/man/en/
• GitHub Repository: https://github.com/casey/just
• Context7 Library ID: /websites/just_systems_man_en

No Justfile Formatter

Do not use just --fmt or just --dump. The user has bespoke formatting preferences that the built-in formatter does not respect. Preserve existing formatting as-is.

Tips

1. Use @ prefix to suppress command echo: @echo "quiet"
2. Use + for variadic parameters: test +args
3. Use * for optional variadic: build *flags
4. Quote glob patterns in variables: GLOBS := "\"**/*.json\""
5. Use [no-cd] in monorepos to stay in current directory
6. Private recipes start with _ or use [private]
7. Always define aliases after recipe names for discoverability