justfile

name	justfile
description	Write correct, idiomatic Justfiles with just. Use when creating a Justfile, writing just recipes, adding project tasks, or mentioning "justfile", "just", or ".just".

When to use

Use this skill when asked to create, modify, or review a Justfile — the task file format for the just command runner.

These reference files are self-contained — prefer them over external sources.

Key Concepts

just is a command runner, NOT a build system. Justfiles define named recipes (tasks) with shell commands, variables, and expressions. Key differences from make: no file targets, no automatic variables, no implicit rules, spaces and tabs both work (but must be consistent per recipe).

Validating justfiles

Run each just command as its own standalone Bash call with no shell additions. NEVER append 2>&1, || echo ..., &&, for loops, or any other compound shell constructs.

just --justfile /path/to/justfile --dump
just --justfile /path/to/justfile --list
just --justfile /path/to/justfile --dry-run recipe-name
just --justfile /path/to/justfile --fmt --check
just --justfile /path/to/justfile --evaluate

--dump prints the formatted justfile to stdout and exits 0 on a valid parse; parse errors go to stderr with a non-zero exit. --dump only checks syntax — it does not evaluate variables, so error() and assert() at variable level are NOT triggered. --evaluate closes that gap by evaluating all top-level variables, so variable-level error()/assert() do trigger — but it also executes backticks and shell() calls, so inspect the justfile for side-effecting backticks before running it.

Critical Rules

• Each linewise recipe line runs in a separate shell. Shell state (cd, variables) does NOT persist between lines. Use shebang or [script] recipes for multi-line logic.
• Use {{variable}} to interpolate just variables in recipe bodies. Bare variable or $variable references shell variables, not just variables.
• {{{{ produces a literal {{ in recipe bodies.
• {{…}} evaluates expressions only — NEVER use it to invoke recipes.
• NEVER mix tabs and spaces within a single recipe's indentation.
• Functions like path_exists(), semver_matches(), and is_dependency() return strings "true"/"false", not booleans — always compare with == "true".
• import merges into the current scope (flat inclusion). mod creates an isolated namespace — parent variables are NOT visible inside modules and vice versa.
• NEVER add set unstable preemptively. Write the justfile without it and validate with --dump. If just reports a feature is unstable, inform the user and get explicit approval before adding set unstable. This makes the installed just version the source of truth — unstable markers in this skill may be outdated.
• Give every public recipe a one-line doc comment (house convention; it becomes the --list description).

Reference Files

File	Start here for...
references/basics.md	Writing a basic justfile — recipes, dependencies, parameters, aliases, comments, line sigils
references/variables-and-expressions.md	Variables, strings, interpolation — assignment, string types, operators, backticks
references/functions.md	Using built-in functions — all ~70 built-in functions by category
references/conditionals-and-flow.md	Conditional logic, error handling — if/else, assert, error(), guards
references/settings.md	Configuring shell, dotenv, quiet mode — all set directives, shell config, dotenv, export
references/attributes.md	Recipe attributes (platform, groups, scripts) — platform targeting, groups, script execution, arg options
references/modules-and-imports.md	Splitting into multiple files — mod, import, submodule isolation, file resolution
references/advanced-patterns.md	Shebang recipes, cross-platform, idioms — shebang/script recipes, cross-platform, constants
references/invocation-primer.md	Understanding how users invoke recipes — CLI invocation, argument passing, flags affecting authoring