Initsystemer

Initsystemer, or init systems, are the first process started by the kernel during the boot process of a Unix-like operating system. Its primary role is to initialize the rest of the system by starting all other necessary processes, often referred to as daemons. The init process is usually assigned process ID 1.

Historically, the most common init system was SysVinit, which uses a series of runlevels to manage system state. Each runlevel corresponds to a specific set of services and daemons that should be running. Transitioning between runlevels involves stopping services in the current runlevel and starting those required for the new one.

More modern init systems, such as systemd and Upstart, have emerged to address perceived limitations of SysVinit. systemd, in particular, has gained widespread adoption and offers a more parallelized approach to service startup, dependency management, and system control. It uses "units" to represent various system resources and manages them through a socket-based activation mechanism. Upstart, developed by Canonical for Ubuntu, also aimed for faster boot times by using an event-driven model.

The choice of init system impacts how services are managed, how the system boots, and how administrative tasks like starting, stopping, and checking the status of daemons are performed. Each system has its own set of commands and configuration file structures, leading to differences in system administration practices across various Linux distributions and Unix-like operating systems.