Once overshadowed by graphical interfaces, the command-line interface (CLI) is now a critical tool across cloud infrastructure, DevOps, SaaS automation, and cybersecurity. Its resurgence is fueled by the need for speed, precision, and automation in a fast-paced, AI-centric software world. CLI design and engineering practices have returned to the forefront. This book is a modern guide to command-line development, written for designers, engineers, architects, and toolmakers building the next generation of CLI applications. It offers clear explanations, battle-tested patterns, and real-world examples written in Go-an ideal language for high-performance, cross-platform development. Readers will also find Spotlights on widely adopted tools like Git(R), WP-CLI, and Warp Terminal(R)-each revealing the design thinking behind some of today's most influential CLI-centric tooling. Additionally, technical programming chapters are supplemented with LLM co-pilot guidance. Whether you're creating developer tools, enterprise-grade utilities, or modern shell-based automation, this book delivers the insight and guidance to build composable, maintainable, and future-ready CLIs that work the way today's teams and systems demand.