Act vs Buck2

act is an open-source command-line tool that runs GitHub Actions workflows locally using Docker containers. It reads workflow files from .github/workflows/ and executes them with environment variables and filesystem configurations matching GitHub's hosted runners. The tool enables fast local feedback on workflow changes without requiring commits and pushes to a remote repository.

Buck2 is Meta's open-source build system, a complete rewrite of the original Buck. It provides fast, reliable, multi-language builds with remote execution support and a Starlark-based configuration language.