Homebrew Formula Development

Guide for researching, creating, testing, and maintaining Homebrew formulas in a custom tap.

When to Use This Skill

• Creating a new Homebrew formula for a project
• Debugging formula build or test failures
• Running local validation before CI
• Understanding Homebrew's Ruby DSL
• Setting up livecheck for automatic version detection

Template Pipeline

This skill includes a JSON Schema → Mustache template pipeline for generating formulas from structured data.

Workflow

1. Create a JSON file conforming to scripts/formula.schema.ts
2. Run just template-formula <path-to-json> to validate and render
3. The pipeline validates with AJV, preprocesses (PascalCase, language dispatch, license rendering), then renders via Mustache

Key Files

File	Purpose
scripts/formula.schema.ts	JSON Schema (draft-2020-12) defining formula structure
scripts/formula.helper.ts	Preprocessing: PascalCase, license rendering, install flattening, partials loading
reference/templates/main.mustache	Main template — renders all shared fields, dispatches to language partials
reference/templates/langs/*.mustache	Language-specific install partials (go, rust, python, zig, cmake, autotools, meson)
test/data/*.json	Test fixtures covering each scenario
test/cases/*.sh	Test cases that validate rendered output

Running

# Render a formula from JSON
just template-formula path/to/formula.json

# Run all tests
just test

Adding a New Language

1. Add install-<lang> definition to scripts/formula.schema.ts
2. Add language dispatch allOf entry in the formula definition
3. Create reference/templates/langs/<lang>.mustache partial
4. Add "<lang>" to the language enum
5. Add a test fixture in test/data/ and test case in test/cases/

Research Phase

Before creating a formula, gather this information:

Field	How to Find
Latest version	gh api repos/owner/repo/releases/latest --jq '.tag_name' (404 → HEAD-only)
License	Check LICENSE file or repo metadata (use SPDX identifier)
Build system	Look at Makefile, go.mod, Cargo.toml, pyproject.toml, etc.
Dependencies	Check build docs, CI files, or dependency manifests
Default branch	Check repo settings — may be main or master
Binary name	May differ from formula name — check Cargo.toml [[bin]], Go cmd/, or pyproject.toml [project.scripts]

Determine Formula Type

Scenario	Type	Has url/sha256?	Has livecheck?
Tagged releases	Standard	Yes	Yes
No releases	HEAD-only	No	No
Monorepo subdirectory	Standard	Yes	Yes

Calculate SHA256

curl -sL "https://github.com/owner/repo/archive/refs/tags/vX.Y.Z.tar.gz" | shasum -a 256

Formula Naming

• Formula name: kebab-case (hex-patch, jwt-ui)
• Class name: PascalCase (HexPatch, JwtUi)

Formula Structure

File Location

Formulas are organized alphabetically: Formula/<first-letter>/<name>.rb

Key Elements

Element	Purpose
desc	Short description (~80 chars) for brew info
homepage	Project homepage URL
url	Source tarball URL (omit for HEAD-only)
sha256	Checksum (omit for HEAD-only)
license	SPDX identifier
head	Git URL for --HEAD installs
livecheck	Auto-detect new versions (omit for HEAD-only)
depends_on	Build or runtime dependencies
test	Verification block

SPDX License Identifiers

License	SPDX
MIT	"MIT"
Apache 2.0	"Apache-2.0"
GPL 3.0 (only)	"GPL-3.0-only"
GPL 3.0 (or later)	"GPL-3.0-or-later"
BSD 2-Clause	"BSD-2-Clause"
BSD 3-Clause	"BSD-3-Clause"

Always specify -only or -or-later for GPL/LGPL/AGPL.

Language-Specific Patterns

Each language has a reference doc with install patterns, schema fields, and common issues:

• Go: reference/langs/go.md
• Rust: reference/langs/rust.md
• Python: reference/langs/python.md

Additional languages supported by the template pipeline (cmake, autotools, meson, zig) — see their Mustache partials in reference/templates/langs/.

Reference Materials

Topic	Location
Local validation steps	reference/checklists/local-validation.md
Common issues & FAQ	reference/faq/common.md
Test block patterns	reference/testing/patterns.md
Generated formula examples	reference/templates/formulas/*.rb
JSON Schema definition	scripts/formula.schema.ts
Bottle attestation & provenance	reference/security/attestation.md

Batch Formula Creation

When creating many formulas at once:

1. Compute SHA256 hashes in parallel — launch multiple curl | shasum calls concurrently
2. Research build details in parallel — check build manifests concurrently
3. Write all formula files — no dependencies between them
4. Create branches/PRs sequentially — one branch per formula, each from main
5. Use ruby -c *.rb to syntax-check all formulas before pushing

Architecture-Specific Binaries

When a project provides pre-built binaries for different architectures:

Preferred: Build from source (avoids architecture complexity)

If pre-built binaries required: Use resource blocks, NOT url/sha256 in on_arm/on_intel:

on_arm do
  resource "binary" do
    url "https://github.com/org/repo/releases/download/vX.Y.Z/tool-darwin-arm64.tar.gz"
    sha256 "..."
  end
end

on_intel do
  resource "binary" do
    url "https://github.com/org/repo/releases/download/vX.Y.Z/tool-darwin-amd64.tar.gz"
    sha256 "..."
  end
end

def install
  resource("binary").stage do
    bin.install "tool"
  end
end

See reference/faq/common.md for details on deprecated patterns.

Common Pitfalls

Dependency Issues

Problem	Symptom	Solution
Missing runtime dependency	brew linkage --test shows broken deps	Add dependency globally, not just in on_linux
Wrong dependency order	brew style fails with ordering error	Build deps (=> :build) before runtime deps, alphabetically within each
Rust + OpenSSL linkage	Linkage test shows libssl.3.dylib broken	Add depends_on "openssl@3" globally (not just Linux) if binary links dynamically

Python Formula Issues

Problem	Symptom	Solution
pip_install buildpath missing deps	ModuleNotFoundError at runtime	Use pip_install "package==#{version}" to install from PyPI
PyPI name differs from repo	pip can't find package	Check pyproject.toml for actual name field (e.g., ktool → k2l)
Missing setuptools	No module named 'pkg_resources'	Add venv.pip_install "setuptools" before main package
Wrong pip invocation	Failed to execute: .../libexec/bin/pip	Use venv.pip_install, not system libexec/"bin/pip"

Test Block Issues

Problem	Symptom	Solution
Binary name mismatch	Test can't find binary	Check Cargo.toml [[bin]] or pyproject.toml [project.scripts] for actual name
Help text mismatch	assert_match fails	Run binary locally to verify actual output before writing test
Output to stderr	shell_output returns empty	Add 2>&1 redirect: shell_output("#{bin}/tool --help 2>&1")

See reference/faq/common.md for detailed explanations and additional issues.

Checklist

• Research complete (version, license, build system, deps, binary name, default branch)
• Formula type determined (standard vs HEAD-only)
• SHA256 calculated (if not HEAD-only)
• Formula file created at Formula/<letter>/<name>.rb
• ruby -c passes (syntax check)
• brew audit --new passes
• brew style passes (or issues addressed)
• brew install --build-from-source succeeds
• brew test passes
• Binary executes correctly
• PR created with CI passing

References

• Homebrew Formula Cookbook
• SPDX License List
• Homebrew Ruby Style Guide
• Homebrew Python Guidelines