This page describes the various init systems which are available as alternatives to systemd
For a more comprehensive explanation of init, and runlevels, see wikipedia.org Page: Init
Init is the first process started during system boot. It is a a daemon process that continues running until the system is shut down. Init is the direct or indirect ancestor of all other processes, and automatically adopts all orphaned processes. It is started by the kernel using a hard-coded filename; if the kernel is unable to start it, panic will result. Init is typically assigned process identifier 1.
The init scripts (or rc) are launched by the init process to guarantee basic functionality on system start and shutdown. This includes (un)mounting of file systems and launching of daemons. A service manager takes this one step further by providing active control over launched processes, or process supervision. An example is to monitor for crashes and restart processes accordingly.
These components combine to the init system. Some inits include the service manager in the init process, or have init scripts in close relation to them. These inits are below referred to as integrated, though entries in different categories may explicitly depend on each other.
A nice (but still incomplete) overview of init systems can be found in this blog entry, titled "A history of modern init systems (1992-2015)"
Alternatives to systemd
init + runlevel configuration (integrated management)
|dinit||init system with process supervision, service dependencies and socket activation written in C++||2017-03-30|
|procd||OpenWrt init and process management daemon with ubus integration||2016-06-20|
|Epoch||Single-threaded init system designed for minimal footprint, compatibility and unified configuration||2015-06-23|
|finit||Fast and extensible init, originally based on EeePC fastinit||2015-10-16|
|initng||Dependency-based init system with parallelization and asynchronous start||2007-11-11|
|eINIT||Can supervise processes and start them asynchronously, possibly without shell scripts||2007-12-18|
|nosh||A suite of system-level utilities for initializing and running a BSD or Linux system, for managing daemons, for managing terminals, and for managing logging||2017-07-05|
|runit||UNIX init scheme with service supervision, a replacement for SysVinit, and other init schemes||2014-08-10|
|Upstart||Event-based init system which handles starting, stopping and supervising of tasks and services. This project is dead||2014-09-04|
|uselessd||A fork of systemd v208. This project is dead||2014-11-16|
|System XVI||Modular, self-healing, and interface-oriented service manager and init system. This project is dead||2015-11-07|
|minit||A very small init with process supervision, service dependency ordering and parallel service activation||2005 ?|
|s6-rc||A service manager for s6-based systems, i.e. a suite of programs that can start and stop services, both long-running daemons and one-time initialization scripts, in the proper order according to a dependency tree||2017-07-04|
|anopa||An init system/service manager built around s6 supervision suite||2017-06-20|
|GNU PIES||Program Invocation and Execution Supervisor (inittab-compatible with a native rc format as well)||2016-10-01|
|OpenRC|| Dependency-based parallel-process startup rc system that works with the system-provided init, normally SysVinit.
Since v0.25, OpenRC includes openrc-init, which can replace /sbin/init, but the default provider for the init command is SysVinit for OpenRC.
standalone init (separate from configuration/management of runlevels)
|busybox init||BusyBox init.||2017-08-17|
|ninit||Fork from minit. See ninit Instructions.||2010-01-16|
|myinit||Simple init with parallel execution and dependencies via reference counting (`start' and `stop' commands).||2011-07-11|
|sinit||Simple init initially based on Rich Felker’s minimal init.||2015-06-16|
|SysVinit||Traditional System V init.||2010-04-14|
|sninit||Small init implementation with SysV init like (sub)runlevels.||2015-12-31|
|uinit||Smallest init possible.||2017-05-16|
|ueld||Similar to BSD init but more integrated.||2017-06-24|
|simpleinit-msb||A fork of simpleinit (shipped with util-linux until v2.20) by Matthias S. Benkmann currently used in Source Mage GNU/Linux.||2007-11-08|
|restartd||Debian restartd - A process-restarting daemon||2013-01-11|
|daemontools||Collection of tools for managing UNIX services.||2001-07-12|
|daemontools encore||enhanced version of daemontools github repository||2014-04-02|
|GNU Shepherd||A service manager written in Guile scheme that is used as PID 1 by GuixSD Linux.||2016-12-04|
|Monit||Monit is a process supervision tool for Unix and Linux. With monit, system status can be viewed directly from the command line, or via the native HTTP(S) web server.||2017-06-07|
|perp||Persistent process (service) supervisor and managment framework for UNIX.||2013-01-11|
|s6||Small suite of programs for UNIX, designed to allow service supervision in the line of daemontools and runit.||2017-06-15|
|freedt||A daemontools reimplementation.||2014-09-03|
|procer||mongrel2's process supervisor procer||2014-03-18|
|god (2)||Easy to configure and extend monitoring framework written in Ruby.||2014-03-06|
|watchman||A not-so-simple service manager for Linux.||2016-03-31|
|Supervisor||A process control system written in Python.||2016-02-06|
|startpar||May be used by the SysV RC boot system executor to allow parallel process system start-up.||2014-02-09|
|insserv||May be used with SysV-based init systems to allow dependency driven system start-up based on dependencies specified by LSB headers in the init.d scripts.||2012-11-14|
SEE ALSO: Gentoo wiki page: Comparison of init systems presents an even more extensive / thorough "Detailed Feature Comparison Table"