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AGENTS.md

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Agent Instructions for Homebrew/brew

Most importantly, run ./bin/brew lgtm to verify any file edits before prompting for input to run all style checks and tests.

This is a Ruby based repository with Bash scripts for faster execution. It is primarily responsible for providing the brew command for the Homebrew package manager. Please follow these guidelines when contributing:

When running commands in this repository, use ./bin/brew (not a system brew on PATH).

Code Standards

Required Before Each Commit

  • Run ./bin/brew typecheck to verify types are declared correctly using Sorbet. Individual files/directories cannot be checked. ./bin/brew typecheck is fast enough to just be run globally every time.
  • Run ./bin/brew style --fix --changed to lint code formatting using RuboCop. Individual files can be checked/fixed by passing them as arguments e.g. ./bin/brew style --fix Library/Homebrew/cmd/reinstall.rb
  • Run ./bin/brew tests --online --changed to ensure that RSpec unit tests are passing (although some online tests may be flaky so can be ignored if they pass on a rerun). Individual test files can be passed with --only e.g. to test Library/Homebrew/cmd/reinstall.rb with Library/Homebrew/test/cmd/reinstall_spec.rb run ./bin/brew tests --only=cmd/reinstall.
  • Shortcut: ./bin/brew lgtm --online runs all of the required checks above in one command.
  • All of the above can be run via the Homebrew MCP Server (launch with ./bin/brew mcp-server).

Sandbox Setup

  • When working in a sandboxed environment e.g. OpenAI Codex, copy the contents of $(./bin/brew --cache)/api/ to a writable location inside the repository (for example, tmp/cache/api/) and export HOMEBREW_CACHE to that writable directory before running ./bin/brew tests. This avoids permission errors when the suite tries to write API cache or runtime logs.

Development Flow

  • Write new code (using Sorbet sig type signatures and typed: strict for new files, but never for RSpec/test/*_spec.rb files)
  • Write new tests (avoid more than one :integration_test per file for speed). Write fast tests by preferring a single expect per unit test and combine expectations in a single test when it is an integration test or has non-trivial before for test setup.
  • Keep comments minimal; prefer self-documenting code through strings, variable names, etc. over more comments.

Repository Structure

  • bin/brew: Homebrew's brew command main Bash entry point script
  • completions/: Generated shell (bash/fish/zsh) completion files. Don't edit directly, regenerate with ./bin/brew generate-man-completions
  • Library/Homebrew/: Homebrew's core Ruby (with a little bash) logic.
  • Library/Homebrew/bundle/: Homebrew's brew bundle command.
  • Library/Homebrew/cask/: Homebrew's Cask classes and DSL.
  • Library/Homebrew/extend/os/: Homebrew's OS-specific (i.e. macOS or Linux) class extension logic.
  • Library/Homebrew/formula.rb: Homebrew's Formula class and DSL.
  • docs/: Documentation for Homebrew users, contributors and maintainers. Consult these for best practices and help.
  • manpages/: Generated man documentation files. Don't edit directly, regenerate with ./bin/brew generate-man-completions
  • package/: Files to generate the macOS .pkg file.

Key Guidelines

  1. Follow Ruby and Bash best practices and idiomatic patterns.
  2. Maintain existing code structure and organisation.
  3. Write unit tests for new functionality.
  4. Document public APIs and complex logic.
  5. Suggest changes to the docs/ folder when appropriate
  6. Follow software principles such as DRY and YAGNI.
  7. Keep diffs as minimal as possible.
  8. Prefer shelling out via HOMEBREW_BREW_FILE instead of requiring cmd/ or dev-cmd when composing brew commands.
  9. Inline new or existing methods as methods or local variables unless they are reused 2+ times or needed for unit tests.