initsysteem

An init system is the first user-space process started by the kernel during the boot sequence. It runs as PID 1 and is responsible for starting and enumerating all other processes and services required for the system to function. The init system orchestrates system initialization, handles service dependencies, and may provide facilities for monitoring, logging, and orderly shutdown.

Historically, many Unix-like systems used SysVinit, which relies on sequential shell scripts and runlevels to determine which services to start. Later, event-driven or parallel-start variants emerged, such as Upstart, which uses events to start services, and systemd, which uses a dependency graph of units to start services in parallel and to offer extensive features such as service supervision and on-demand activation. Other init systems include OpenRC, runit, s6, and nosh. Different distributions adopt different implementations; for example, Debian, Fedora, and many others use systemd as the default, while some distributions use OpenRC or runit.

Key concepts include units or services, targets or runlevels, dependency management, and the ability to restart, reload, or isolate services without rebooting. Modern init systems often support parallel startup to reduce boot time, service supervision to recover failed processes, cgroup-based resource management, and snapshot/restoration of service states.

Critics point to concerns about complexity and portability across systems, while proponents cite faster boot times, robustness, and unified management. See also: PID 1, runlevel, target, service manager, unit.