Guix System Distribution

GNU Guix

GuixSD is based on GNU Guix, a purely functional package and system configuration manager derived from Nix, using the GNU Guile implementation of the Scheme programming language. All package recipes, as well as the whole system configuration, are written in declarative embedded domain-specific languages in Guile Scheme. The package and system management is purely functional in the sense that the output of evaluating a package recipe or system configuration persistently yields the same results, without interference of the state of the software environment under which these evaluations are done.

GNU Shepherd

GuixSD uses the GNU Daemon Shepherd as its init system, which is developed in tandem with Guix and is written in Guile as well. It was previously known as "dmd", which stood for "Daemon managing Daemons" or "Daemons-managing Daemon", but changed names to avoid collision with another project.

Shepherd takes some inspiration from systemd, another recent init system, in supplying user space functionality asynchronously as services, which under Shepherd are generic functions and object data types that are exported for use by the Shepherd to extend the base operating system in some defined way. Core to the Shepherd model of user space initialisation is the concept of the extension, a form of composability where services are designed to be layered onto other services, augmenting them with more elaborate or specialised behaviours as desired. This expresses the instantiation-based dependency relationships found in many modern init systems, making the system modular, but also allows services to interact variadically with other services in arbitrary ways.

Shepherd also provides so-called virtual services which allow dynamic dispatch over a class of related service objects, such as all those which instantiate an MTA for the system. A system governed via the Shepherd daemon can represent its user space as a directed acyclic graph, with the "system-service" − responsible for early phases of boot and init − as its root, and all subsequently initialised services as extensions to system-service's functionality, either directly or over other services.

Being both written and configured in Guile Scheme, GNU Shepherd is intended to be highly programmable by the system administrator, but it can also be used to manage per-user profiles of unprivileged daemons and services. Its services and configuration are stored uniformly as object-oriented Scheme code, and while a core set of services are provided with the basic Guix System Distribution, arbitrary new services can be flexibly declared, and through Guile's object system, GOOPS, existing services can be redefined at the user's discretion by asking the Shepherd to dynamically rewrite services in specified ways on instantiation.

GNU Shepherd was originally designed to work with GNU Hurd, and was later adopted by GuixSD.