Justfile

Just

Use this skill when a repo may expose commands through justfile.

Treat just as the canonical task interface when it exists. Prefer existing recipes over reconstructing the underlying go, npm, cargo, docker, make, or shell commands by hand.

When To Use This Skill

Use it when:

• the repo or a nearby subproject has a justfile
• the user asks how to build, test, lint, run, deploy, or inspect commands
• the user mentions just, justfile, recipes, or recipe arguments
• the repo has both Makefile and justfile and you need to understand which command surface is intended
• the user wants to migrate from make to just

Default Workflow

1. Find the nearest relevant justfile. Do not assume repo root.
2. Run just --list in that directory to discover the supported UX.
3. If a recipe is unclear, inspect it with just --show <recipe>.
4. Use the recipe directly unless the user explicitly wants the raw command.
5. Fall back to ad hoc commands only when no suitable recipe exists.

Common recipe names to check first: setup, install, build, test, lint, fmt, dev, run, preview, deploy.

Agent Guardrails

• Prefer the nearest task-specific justfile over a broader root-level one.
• If both Makefile and justfile exist, inspect both before suggesting migration or replacement.
• Do not bypass an existing recipe unless it is missing, clearly wrong for the task, or the user asks for the underlying command.
• Do not assume .env exists, and do not print secrets just because a justfile uses dotenv-load or exported variables.
• Treat destructive or confirmation-gated recipes cautiously.
• Test-like shebang recipes (test, test-*, etc.) should use #!/usr/bin/env bash and set -euo pipefail so failures propagate instead of being masked by later commands.

Common Failure Modes

Wrong Working Directory

Nested justfiles are common. If commands look wrong, verify you are running in the correct subproject. When needed, invoke a specific file directly with just --justfile path/to/justfile <recipe> instead of guessing.

Sparse Or Misleading --list Output

Some repos split recipes across imported files or modules. If just --list looks too small, inspect the justfile for import or mod statements before assuming recipes are missing.

Hidden Helper Recipes

[private] recipes do not appear in just --list or just --summary, so they do not pollute help output. They are still valid recipe entry points and can be invoked directly with just <private-recipe> when an internal workflow, CI job, or explicit user request needs them. Check just --show <recipe>, use just --dry-run <recipe>, or inspect the file if a public recipe depends on helpers you cannot see in the listing.

Env Or Shell Drift

Behavior can change based on settings such as:

• set dotenv-load
• set export
• set shell := [...]
• set positional-arguments

If a recipe works differently in CI, in a subdirectory, or in the terminal, check these settings before changing the command.

Confirmation Gates

Recipes marked with [confirm] may block or fail in non-interactive contexts. Treat them as intentional safety rails, especially for destroy, clean, or production deploy flows.

Quick Command Reference

• just --list list public recipes with descriptions
• just --show <recipe> show the resolved recipe body
• just --summary list recipe names only
• just --dry-run <recipe> print commands without executing them, including private recipes
• just --dump print the full justfile
• just --evaluate inspect variable evaluation
• just <recipe> [args...] run a recipe
• just --justfile path/to/justfile <recipe> target a specific justfile

Notes

• Comments above recipes become descriptions in just --list.
• Each recipe line runs in a separate shell unless the recipe is a shebang script.
• Large repos may use just as a command hub over other toolchains rather than exposing the toolchain directly.

References

• references/make-migration.md
• references/justfile-patterns.md