Justfile Expert

Justfile Expert

Expert knowledge for Just command runner, recipe development, and task automation with focus on cross-platform compatibility and project standardization.

When to Use This Skill

Core Expertise

Command Runner Mastery

• Justfile syntax and recipe structure
• Cross-platform task automation (Linux, macOS, Windows)
• Parameter handling and argument forwarding
• Module organization for large projects

Recipe Development Excellence

• Recipe patterns for common operations
• Dependency management between recipes
• Shebang recipes for complex logic
• Environment variable integration

Project Standardization

• Golden template with standard naming and section structure
• Self-documenting project operations
• Portable patterns across projects
• Integration with CI/CD pipelines

Recipe Naming Conventions

Semantic Workflow Recipes

Standard composite recipes with defined meanings:

# Composite: code quality only (no tests)
check: format-check lint typecheck

# Pre-commit checks (fast, non-mutating)
pre-commit: format-check lint typecheck test-unit
    @echo "Pre-commit checks passed"

# Full CI simulation
ci: check test-coverage build
    @echo "CI simulation passed"

# Clean build artifacts
clean:
    rm -rf dist build .next

# Clean everything including deps
clean-all: clean
    rm -rf node_modules .venv __pycache__

Key Capabilities

Recipe Parameters

• Required parameters: recipe param: - must be provided
• Default values: recipe param="default": - optional with fallback
• Variadic +: recipe +FILES: - one or more arguments
• Variadic *: recipe *FLAGS: - zero or more arguments
• Environment export: recipe $VAR: - parameter as env var

Settings Configuration

• set dotenv-load: Load .env file automatically
• set positional-arguments: Enable $1, $2 syntax
• set export: Export all variables as env vars
• set shell: Custom shell interpreter
• set quiet: Suppress command echoing

Recipe Attributes

• [doc("text")]: The --list description. Overrides the comment above the recipe; bare [doc] suppresses it. See "What --list Shows" below — without this attribute only the comment block's LAST line is used
• [private]: Hide from --list and --summary output
• [no-cd]: Don't change directory
• [no-exit-message]: Suppress exit messages
• [unix] / [windows] / [linux] / [macos]: Platform-specific recipes
• [positional-arguments]: Per-recipe positional args
• [confirm] / [confirm("message")]: Require confirmation before running
• [group: "name"] / [group("name")]: Section recipes in --list; both spellings work, and --groups lists the group names
• [working-directory: "path"]: Run in specific directory

Module System

• mod name: Declare submodule
• mod name 'path': Custom module path
• Invocation: just module::recipe or just module recipe
• set fallback is NOT inherited by a module. The parent may fall through to its parent, but just sub::parent-recipe fails with justfile does not contain recipe. A module's recipes resolve only within that module

Essential Syntax

Basic Recipe Structure

# Comment describes the recipe
recipe-name:
    command1
    command2

Recipe with Parameters

build target:
    @echo "Building {{target}}..."
    cd {{quote(target)}} && make

test *args:
    uv run pytest {{args}}

Interpolation is UNQUOTED — quote anything that can contain spaces

{{...}} splices raw text into the recipe body before the shell parses it, so a value carrying spaces or quotes word-splits. This bites hardest on the *args passthrough above, because the error is reported by the called program rather than by just, which makes it read like a bug in the tool:

# Trap — one argument with spaces arrives as several
caption *ARGS:
    ./tool.py {{ARGS}}

$ just caption ./data "the subject's face"
tool.py: error: unrecognized arguments: subjects face

The outer shell consumed the quotes (taking the apostrophe with them) and the / subject's / face arrived as three separate argv entries. Name the parameters that can contain spaces and run them through quote(), which emits a properly shell-escaped literal:

# Correct — named params are quoted; trailing flags still pass through
caption DIR SUBJECT="" *ARGS:
    ./tool.py {{quote(DIR)}} {{quote(SUBJECT)}} {{ARGS}}

quote() covers embedded spaces, ', ", and $. Keep {{ARGS}} bare — that is what lets several trailing flags expand as separate words — and accept its corollary: an individual passthrough flag's value must not contain spaces. When one might, promote it to a named parameter too.

What --list Shows Is ONE Line, and It Is Not Your Comment Block

just --list renders a single description per recipe. With no [doc] attribute it takes the last line of the comment block immediately above the recipe — not the first line, and not the block:

So "add a comment before each recipe" is not the same as documenting it. A block that ends in an example or a caveat — the normal way to write one — lists as that fragment:

# Pitch-correct the singing in an MP4. Video is stream-copied.
#   just autotune take.mp4 out.mp4 --key C:minor
autotune IN OUT *FLAGS:

$ just --list
    autotune IN OUT *FLAGS   # just autotune take.mp4 out.mp4 --key C:minor

Add [doc("one line")] as soon as a recipe's comment block exceeds one line. The block stays where it is and keeps carrying the detail; the attribute is the only thing --list reads.

The block binds by ADJACENCY, and reassignment is silent. A blank line ends a block, so inserting a recipe between a block and the recipe it describes hands the block to the newcomer — the original then lists blank, and nothing warns. Re-read just --list after inserting a recipe into an existing file.

A recipe with a required positional has no --help form. recipe *ARGS: forwards --help to the underlying tool, but just refuses the call before the tool runs once a positional is required:

$ just autotune --help
error: recipe `autotune` got 1 positional argument but takes at least 2

There is no bare-help spelling for such a recipe. Put the flags in its [doc] or comment block, or add a help recipe that prints them.

Recipe Dependencies

default: build test

build: _setup
    cargo build --release

_setup:
    @echo "Setting up..."

Variables and Interpolation

version := "1.0.0"
project := env('PROJECT_NAME', 'default')

info:
    @echo "Project: {{project}} v{{version}}"

Conditional Recipes

[unix]
open:
    xdg-open http://localhost:8080

[windows]
open:
    start http://localhost:8080

Standard Recipes

Every project should provide these standard recipes, organized by section:

# Justfile - Project task runner
# Run `just` or `just help` to see available recipes

set dotenv-load
set positional-arguments

# Default recipe - show help
default:
    @just --list

# Show available recipes with descriptions
help:
    @just --list --unsorted

####################
# Development
####################

# Start development environment
dev:
    # bun run dev / uv run uvicorn app:app --reload / skaffold dev

# Build for production
build:
    # bun run build / cargo build --release / docker build

# Clean build artifacts
clean:
    # rm -rf dist build .next

####################
# Code Quality
####################

# Run linter (read-only)
lint *args:
    # bun run lint / uv run ruff check {{args}}

# Auto-fix lint issues
lint-fix:
    # bun run lint:fix / uv run ruff check --fix .

# Format code (mutating)
format *args:
    # bun run format / uv run ruff format {{args}}

# Check formatting without modifying (non-mutating)
format-check *args:
    # bun run format:check / uv run ruff format --check {{args}}

# Type checking
typecheck:
    # bunx tsc --noEmit / uv run basedpyright

####################
# Testing
####################

# Run all tests
test *args:
    # bun test {{args}} / uv run pytest {{args}}

# Run unit tests only
test-unit *args:
    # bun test --grep unit {{args}} / uv run pytest -m unit {{args}}

####################
# Workflows
####################

# Composite: code quality (no tests)
check: format-check lint typecheck

# Pre-commit checks (fast, non-mutating)
pre-commit: format-check lint typecheck test-unit
    @echo "Pre-commit checks passed"

# Full CI simulation
ci: check test-coverage build
    @echo "CI simulation passed"

Section Structure

Organize recipes into these standard sections:

Use #################### comment blocks as section dividers for readability.

Common Patterns

Setup/Bootstrap Recipe

# Initial project setup
setup:
    #!/usr/bin/env bash
    set -euo pipefail
    echo "Installing dependencies..."
    uv sync
    echo "Setting up pre-commit..."
    pre-commit install
    echo "Done!"

Docker Integration

# Build container image
docker-build tag="latest":
    docker build -t {{project}}:{{tag}} .

# Run container
docker-run tag="latest" *args:
    docker run --rm -it {{project}}:{{tag}} {{args}}

# Push to registry
docker-push tag="latest":
    docker push {{registry}}/{{project}}:{{tag}}

Database Operations

# Run database migrations
db-migrate:
    uv run alembic upgrade head

# Create new migration
db-revision message:
    uv run alembic revision --autogenerate -m "{{message}}"

# Reset database
db-reset:
    uv run alembic downgrade base
    uv run alembic upgrade head

CI/CD Recipes

# Full CI check (lint + test + build)
ci: lint test build
    @echo "CI passed!"

# Release workflow
release version:
    git tag -a "v{{version}}" -m "Release {{version}}"
    git push origin "v{{version}}"

Shared imports + modules: pass per-project values as recipe parameters

When a monorepo registers submodules (mod name 'path') whose justfiles import a shared recipe file, hand per-project values to the shared recipes as recipe parameters — not via a shared variable. Two just behaviours make the variable approach fail:

• An import that defaults a variable a module also assigns is a conflict, not an override: error: variable X has multiple definitions.
• An imported recipe that references {{X}} is resolved at load time, so it forces every importing module to define X (else error: variable X not defined) — even modules that never run that recipe.

Passing the value as a recipe argument sidesteps both and keeps it explicit at the call site:

# shared.just — take the value as a parameter, not a shared variable
[private]
_flash bin:
    esptool ... 0x10000 build/{{bin}}.bin

# project justfile
import 'shared.just'
bin_name := "my-app"          # this module's own variable
flash: (_flash bin_name)      # pass it as an argument

MCP Integration (just-mcp)

The just-mcp MCP server enables AI assistants to discover and execute justfile recipes through the Model Context Protocol, reducing context waste since the AI doesn't need to read the full justfile.

Installation:

# Via npm
npx just-mcp --stdio

# Via pip/uvx
uvx just-mcp --stdio

# Via cargo
cargo install just-mcp

Claude Desktop configuration (.claude/mcp.json):

{
  "mcpServers": {
    "just-mcp": {
      "command": "npx",
      "args": ["-y", "just-mcp", "--stdio"]
    }
  }
}

Available MCP Tools:

• list_recipes - Discover all recipes and parameters
• run_recipe - Execute a recipe with arguments
• get_recipe_info - Get detailed recipe documentation
• validate_justfile - Check for syntax errors

Agentic Optimizations

Best Practices

Recipe Development Workflow

1. Name clearly: Use descriptive, verb-based names (build, test, deploy)
2. Document what --list reads: a one-line comment is enough; anything longer needs [doc("...")], or the listing shows only the block's last line
3. Use defaults: Provide sensible default parameter values
4. Group logically: section comments for the file, [group("name")] for the listing — a flat --list stops being scannable somewhere around 20 recipes
5. Hide internals: Mark helper recipes as [private]
6. Test portability: Verify on all target platforms

Critical Guidelines

• Always provide default recipe pointing to help
• Use @ prefix to suppress command echo when appropriate
• Use shebang recipes for multi-line logic
• Prefer set dotenv-load for configuration
• Use modules for large projects (>20 recipes)
• Give a recipe a [doc("...")] when its comment block's last line would not read as a description on its own — --list shows only that line (see "What --list Shows" above). To find them: python3 "${CLAUDE_PLUGIN_ROOT}/scripts/just-recipe-help.py" --audit
• Include variadic *args for passthrough flexibility
• Quote all variables in shell commands — {{...}} interpolates unquoted, so wrap any parameter that can contain spaces in quote() (see "Interpolation is UNQUOTED" above); bare {{args}} is correct only for space-free passthrough flags

Comparison with Alternatives

When to use Just:

• Cross-project standard recipes
• Simple, readable task automation
• No tool version management needed

When to use mise tasks:

• Project-specific with tool version pinning
• Already using mise for tool management

When to use Make:

• Legacy projects with existing Makefiles
• Build systems requiring incremental compilation

For the golden justfile template, detailed syntax reference, advanced patterns, and troubleshooting, see REFERENCE.md.