The core API module that composes distinct concerns (commands, filesystem, environment, text) into a unified interface. Demonstrates excellent separation of concerns where each submodule handles a specific responsibility, allowing for independent usage and easy extension. The modular design enables developers to use only the parts they need while maintaining a consistent API surface.

A collection of process launchers (spawn, in-process, debug) implementing a common interface for executing and managing external processes. Shows strong abstraction of process lifecycle management, output capture, and platform-specific handling. The strategy pattern allows switching between different execution mechanisms without changing consumer code, making it reusable in any Ruby application needing controlled process execution.

Extensive library of composable matchers for validating file system state, command output, and process behavior. Examples include have_file_content, be_an_existing_file, and have_exit_status. These matchers follow a consistent pattern, provide detailed failure messages, and can be used independently of the Aruba framework in other testing contexts. The matcher system exemplifies good test DSL design with readability and expressiveness.

Platform detection and delegation mechanism that selects appropriate Unix or Windows implementations based on the running environment. Encapsulates platform-specific differences in command execution, environment variable handling, and file system operations. This abstraction layer allows the core logic to remain platform-agnostic while delegating OS-specific concerns to specialized implementations, demonstrating a clean adapter pattern.