This page describes the various init systems which are available as alternatives to systemd
Init is the first process started during system boot. It is 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 (aka rc runtime configuration scripts) 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 init systems incorporate the service manager in the init process or have init scripts in close relation to them. Below, such init systems are referred to as integrated, although entries in different categories may explicitly depend on each other.
A nice (but still non-comprehensive) overview of init systems can be found in this blog entry, titled “A history of modern init systems (1992-2015)“
|BusyBox init||quite small init without runlevels, solely signal driven IPC (no fifos, sockets, SysV IPC)||2019-10-25|
|dinit||dependency based C++ init with process supervision, roll-back, and socket activation||2020-01-01|
|finit||fast, event based, modular, extensible (C hooks/plugins) init with SysV runlevels, proper daemon supervision and logging, and optional builtin getty and inetd||2018-01-23|
|nosh||exec chaining, dependency based daemontools family C++ init and supervision suite with reliable logging, (console) virtual terminal management, systemd unit file compatibility; can also be used as service manager under a different init||2019-03-17|
|openrc-init||init system of OpenRC, shipped since version 0.25||2019-08-20|
|pies||dependency based GNU init daemon and super server with SysV runlevels, daemon supervision and logging, inetd functionality, socket activation, fine grained per service access control, understands SysV inittab, inetd.conf, and MeTA1 config files||2019-07-02|
|procd (2)||OpenWrt init and process/service management daemon with ubus integration||2018-07-30|
|runit-init||init from runit||2014-08-10|
|sinit||Simple init initially based on Rich Felker’s minimal init||2018-03-26|
|sninit||Small init implementation with SysV init like (sub)runlevels||2017-12-29|
|SysV init (2)||Traditional System V init. New release: v2.96 on July 07 2019||2019-09-11|
|ToyBox init (2)||similar to/(almost) compatible with BusyBox init (same config syntax/file)||2019-10-28|
|ToyBox oneit (2)||very simple init launcher (just (re)spawns a single child process and reacts to incoming signals)||2019-10-28|
|ueld||simple configuration, solely signal driven (like BSD and Busy/ToyBox init)||2018-07-09|
|uinit||Smallest init possible||2017-05-16|
service manager: a suite of programs which start and stop services, both long-running daemons and one-time initialization scripts, in the proper order according to a dependency tree
|66||a collection of system tools built around s6 and s6-rc created to make the implementation and manipulation of service files easier||2019-12-16|
|Circus (docs)||runs, controls and monitors processes and sockets; written in Python; uses ZeroMQ||2019-12-27|
|daemontools encore||an enhanced version of daemontools (github repository)||2018-10-14|
|GNU Shepherd||service manager and init daemon written in Guile (formerly known as GNU dmd)||2019-05-11|
|MiniBase||small userspace utilities for Linux; includes a process supervisor that can be used as stage 2 of process #1 via exec chaining akin to s6-svscan||2019-11-14|
|OpenRC (2, 3)||dependency based service management suite; includes a process supervisor and a simple init alternative||2018-08-06|
|runit||process supervision suite in the line of daemontools; includes a simple init (2) replacement||2014-08-10|
|s6||small process supervision suite for UNIX ala daemontools, runit, and perp; can be used as stage 2 of process #1 via exec chaining||2019-10-21|
|s6-rc||service management suite for s6||2019-21-21|
|Supervisor||process control daemon written in Python||2018-02-15|
|ssm||simple service manager bash scripts for Linux||2018-03-06|
|supervise-daemon from OpenRC||superise-daemon introduced in OpenRC .0.21||2019-08-20|
|Supervision Framework||OpenRC-like service management suite for daemontools(-encore), runit, and s6||2018-09-01|
|systemctl-replacement||drop-in reading systemd service descriptions, with docker-oriented init function, written in Python||2019-10-24|
udev (userspace /dev) is a device manager for the Linux kernel.
In April 2012, udev's source code was merged into the systemd source tree.
Although udev can still be compiled for usage without systemd, Lennart Poettering said that they will not polish udevd outside of systemd, adding:
”Yes, udev on non-systemd systems is in our eyes a dead end, in case you haven't noticed it yet. I am looking forward to the day when we can drop that support entirely.” So to be on the safe side, you probably want to use an alternative to udev:
|eudev (2, 3)||Gentoo's maintained fork of udev||2017-11-22|
|mdev||the plug-and-play manager built into BusyBox||2018-07-31|
|mdevd (github)||nonforking Linux kernel uevent handler daemon; uses the mdev config file format||2018-08-14|
|nldev (2)||Suckless netlink frontend for mdev; replacements for udevd and udevadm (see also nlmon)||2018-08-03|
|smdev||mostly mdev-compatible Suckless program to manage device nodes||2015-04-12|
For more see this StackExchange question.
Related helper tool: udevil (2) (un)mounts removable devices without a password, shows device info, and monitors device changes. It can also mount ISO files, nfs://, smb://, ftp:, ssh:// and WebDAV URLs, and tmpfs/ramfs filesystems. Uses Glib and libudev (i. e. requires (e)udev) and can work completely without systemd, consolekit, policykit, dbus, udisks, gvfs & fuse (although it can coexist with any of these).