Justfile Expert

---
created: 2025-12-16
modified: 2026-08-15
reviewed: 2026-02-06
name: justfile-expert
description: Just command runner expertise — Justfile syntax, recipes, parameters, modules, shebang recipes. Use when authoring justfiles, project commands, or task automation.
user-invocable: false
allowed-tools: Bash, Grep, Glob, Read, Write, Edit, TodoWrite
model: sonnet
---

# 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

| Use this skill when... | Use alternative when... |
|------------------------|------------------------|
| Creating/editing justfiles for task automation | Need build system with incremental compilation → Make |
| Writing cross-platform project commands | Need tool version management bundled → mise tasks |
| Adding shebang recipes (Python, Node, Ruby, etc.) | Already using mise for all project tooling |
| Configuring dotenv loading and settings | Authoring the shell itself (pipes, traps, arg parsing) → `shell-expert` |
| Setting up CI/CD with just recipes | Project already has extensive Makefile |
| Standardizing recipes across projects | Exposing a module for bulk smoke-testing → `cli-smoke-recipes` |

## 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

| Rule | Pattern | Examples |
|------|---------|---------|
| Hyphen-separated | `word-word` | `test-unit`, `format-check` |
| Verb-first (actions) | `verb-object` | `lint`, `build`, `clean` |
| Noun-first (categories) | `noun-verb` | `db-migrate`, `docs-serve` |
| Private prefix | `_name` | `_generate-secrets`, `_setup` |
| `-check` suffix | Read-only verification | `format-check` |
| `-fix` suffix | Auto-correction | `lint-fix`, `check-fix` |
| `-watch` suffix | Watch mode | `test-watch`, `docs-watch` |
| Modifiers after base | `base-modifier` | `build-release` (not `release-build`) |

## Semantic Workflow Recipes

Standard composite recipes with defined meanings:

| Recipe | Composition | Purpose |
|--------|-------------|---------|
| `check` | `format-check` + `lint` + `typecheck` | Code quality only, no tests |
| `pre-commit` | `format-check` + `lint` + `typecheck` + `test-unit` | Fast, non-mutating validation |
| `ci` | `check` + `test-coverage` + `build` | Full CI simulation |
| `clean` | Remove build artifacts | Partial cleanup |
| `clean-all` | `clean` + remove deps/caches | Full cleanup |

```just
# 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**
- **`[private]`**: Hide from `--list` 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 recipes in `--list` output
- **`[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`

## Essential Syntax

**Basic Recipe Structure**
```just
# Comment describes the recipe
recipe-name:
    command1
    command2
```

**Recipe with Parameters**
```just
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:

```just
# 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:

```just
# 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.

**Recipe Dependencies**
```just
default: build test

build: _setup
    cargo build --release

_setup:
    @echo "Setting up..."
```

**Variables and Interpolation**
```just
version := "1.0.0"
project := env('PROJECT_NAME', 'default')

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

**Conditional Recipes**
```just
[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:

```just
# 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:

| Section | Recipes | Purpose |
|---------|---------|---------|
| **Metadata** | `default`, `help` | Discovery and navigation |
| **Development** | `dev`, `build`, `clean`, `start`, `stop` | Core dev cycle |
| **Code Quality** | `lint`, `lint-fix`, `format`, `format-check`, `typecheck` | Code standards |
| **Testing** | `test`, `test-unit`, `test-integration`, `test-e2e`, `test-watch` | Test tiers |
| **Workflows** | `check`, `pre-commit`, `ci` | Composite operations |
| **Dependencies** | `install`, `update` | Package management |
| **Database** | `db-migrate`, `db-seed`, `db-reset` | Data operations |
| **Kubernetes** | `skaffold`, `dev-k8s` | Container orchestration |
| **Documentation** | `docs`, `docs-serve` | Project docs |

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

## Common Patterns

**Setup/Bootstrap Recipe**
```just
# 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**
```just
# 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**
```just
# 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**
```just
# 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:

```just
# 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:**
```bash
# 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`):**
```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

| Context | Command |
|---------|---------|
| List all recipes | `just --list` or `just -l` |
| Dry run (preview) | `just --dry-run recipe` |
| Show variables | `just --evaluate` |
| JSON recipe list | `just --dump --dump-format json` |
| Verbose execution | `just --verbose recipe` |
| Specific justfile | `just --justfile path recipe` |
| Working directory | `just --working-directory path recipe` |
| Choose interactively | `just --choose` |

## Best Practices

**Recipe Development Workflow**
1. **Name clearly**: Use descriptive, verb-based names (`build`, `test`, `deploy`)
2. **Document always**: Add comment before each recipe
3. **Use defaults**: Provide sensible default parameter values
4. **Group logically**: Organize with section comments
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)
- 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

| Feature | Just | Make | mise tasks |
|---------|------|------|------------|
| Syntax | Simple, clear | Complex, tabs required | YAML |
| Dependencies | Built-in | Built-in | Manual |
| Parameters | Full support | Limited | Full support |
| Cross-platform | Excellent | Good | Excellent |
| Tool versions | No | No | Yes |
| Error messages | Clear | Cryptic | Clear |
| Installation | Single binary | Pre-installed | Requires mise |

**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](REFERENCE.md).