Unix has evolved over five decades, shaping modern operating systems, key software technologies, and development practices. Studying the evolution of this remarkable system from an architectural perspective can provide insights on how to manage the growth of large, complex, and long-lived software systems. Along main Unix releases leading to the FreeBSD lineage we examine core architectural design decisions, the number of features, and code complexity, based on the analysis of source code, reference documentation, and related publications. We see that the growth in size has been uniform, with some notable outliers, while cyclomatic complexity has been religiously safeguarded. A large number of Unix-defining design decisions were implemented right from the very early beginning, with most of them still playing a major role. Unix continues to evolve from an architectural perspective, but the rate of architectural innovation has slowed down over the system’s lifetime. Architectural technical debt has accrued in the forms of functionality duplication and unused facilities, but in terms of cyclomatic complexity it is systematically being paid back through what appears to be a self-correcting process. Some unsung architectural forces that shaped Unix are the emphasis on conventions over rigid enforcement, the drive for portability, a sophisticated ecosystem of other operating systems and development organizations, and the emergence of a federated architecture, often through the adoption of third-party subsystems. These findings allow us to form an initial theory on the architecture evolution of large, complex operating system software.

Diomidis Spinellis is Professor of Software Engineering, Head of the Department of Management Science and Technology, and director of the Business Analytics Laboratory (BALab), at the Athens University of Economics and Business, Greece. His research interests include software engineering, IT security, and computing systems. He has written two award-winning, widely-translated books: Code Reading and Code Quality: The Open Source Perspective. In 2016 he published the book Effective Debugging: 66 Specific Ways to Debug Software and Systems. Dr. Spinellis has also published more than 300 technical papers in journals and refereed conference proceedings, which have received more than 8000 citations. He served for a decade as a member of the IEEE Software editorial board, authoring the regular “Tools of the Trade” column and for four years at the magazine’s Editor in Chief. He has contributed code that ships with Apple’s macOS and BSD Unix and is the developer of CScout, UMLGraph, dgsh, and other open-source software packages, libraries, and tools. He holds an MEng in Software Engineering and a PhD in Computer Science, both from Imperial College London. Dr. Spinellis has served as an elected member of the IEEE Computer Society Board of Governors (2013–2015), and is a senior member of the ACM and the IEEE.