From Gentoo Wiki
An init system is the first program, other than the kernel, to be run after a Linux distribution is booted.
Due to the flexibility of Gentoo, several init systems are available. Be aware, however, that even if another init system has been installed and set up, quite often OpenRC is still required for other purposes.
|openrc-init||sys-apps/openrc||An init system (replacement for /sbin/init) included with OpenRC since version 0.25.0.|
|runit||sys-process/runit||a daemontools-inspired process supervision suite that also provides a program suitable for running as process 1.|
|s6 + s6-rc||
|an init system built using components from the s6, s6-rc and s6-linux-init packages|
|sysvinit||sys-apps/sysvinit||a collection of System V-style init programs originally written by Miquel van Smoorenburg|
|systemd||sys-apps/systemd||a modern SysV-style init and rc replacement for Linux systems.|
- Comparison of init systems — compares and contrasts init systems.