GNUHurd

GNU Hurd is the collection of server software that, together with the Mach microkernel, forms the GNU project's Unix-like operating system. It aims to be a complete, free software replacement for Unix, built entirely from free components.

Architecture: Hurd uses a multiserver architecture in which the kernel provides only basic primitives such as IPC and resource management, while system services run as separate user-space processes. File systems, network protocols, password and authentication services, and device drivers are implemented as independent servers. A capability-based model governs access to resources, and the namespace is assembled from translator components that can mount file systems and present data in flexible ways.

Development and status: Development began in the 1990s as a free alternative to Unix. Progress has been slow, and the project has remained experimental rather than production-ready. Debian GNU/Hurd and other ports exist, but hardware support and ease of installation are limited compared with more common operating systems.

Licensing and ecosystem: The Hurd and its user-space components are released under free software licenses, primarily the GNU General Public License, with other licenses as appropriate. It runs on top of the Mach family of microkernels; GNU Mach is a widely used reference implementation.

Significance: The Hurd embodies the GNU project's emphasis on freedom, modularity, and security through separation of concerns, and it illustrates an alternative path to Unix-like operating system design based on a multiserver, capability-based model.