(en.wikipedia.org) init - Wikipedia

ROAM_REFS: https://en.wikipedia.org/wiki/Init

In Unix-based computer operating systems, init (short for initialization) is the first process started during booting of the operating system. Init is a daemon process that continues running until the system is shut down. It is the direct or indirect ancestor of all other processes and automatically adopts all orphaned processes. Init is started by the kernel during the booting process; a kernel panic will occur if the kernel is unable to start it, or it should die for any reason. Init is typically assigned process identifier.

In Unix systems such as System III and System V, the design of init has diverged from the functionality provided by the init in Research Unix and its BSD derivatives. Up until the early 2010s, most Linux distributions employed a traditional init that was somewhat compatible with System V, while some distributions such as Slackware use BSD-style startup scripts, and other distributions such as Gentoo have their own customized versions.

Since then, several additional init implementations have been created, attempting to address design limitations in the traditional versions. These include launchd, the Service Management Facility, systemd, Runit and OpenRC.