| 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 |
