Alternatives to systemd
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 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)"
init systems
| System | Description | Latest release |
|---|---|---|
| BusyBox init (2) | quite small and simple conventional init without runlevels, solely signal driven IPC (no fifos, sockets, SysV IPC); part of BusyBox | 2018-12-30 |
| dinit | dependency based C++ init with process supervision, roll-back, and socket activation | 2018-07-12 |
| epoch (2) | sequential, non-parallel init without dependency tracking designed for minimal footprint and unified configuration | 2015-06-23 |
| 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 |
| initng (2) | modular, extensible, parallel, asynchronous, dependency based init | 2007-03-25 |
| myinit | parallel init with dependency tracking via reference counting (akin to the "need" concept) lacking proper supervision (respawn only, no output logging) | 2011-07-11 |
| 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 | 2017-12-11 |
| oneit (2) | very simple init launcher (just (re)spawns a single child process and reacts to incoming signals) included in ToyBox | 2019-02-09 |
| 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 | 2016-10-01 |
| procd | OpenWrt init and process/service management daemon with ubus integration | 2018-07-30 |
| Pygos init | Small and quite simple init with task/service ordering used in Pygos GNU/Linux (config and init script examples) | 2018-11-25 |
| s6-linux-init (2) | generates an exec-chaining init around s6-svscan | 2018-03-26 |
| sinit | Simple init initially based on Rich Felker’s minimal init | 2015-06-16 |
| sninit | Small init implementation with SysV init like (sub)runlevels | 2015-12-31 |
| SysV init (2) | the traditional Linux System V init clone | 2019-02-25 |
| ToyBox init (2) | small and simple conventional init without runlevels included in ToyBox; solely signal driven; (almost) compatible with BusyBox init (same config syntax/file) | 2019-02-09 |
| ueld | simple configuration, solely signal driven (akin to BSD/BusyBox/ToyBox init) | 2017-06-24 |
| uinit | Smallest init possible | 2017-05-16 |
abandoned/defunct/discontinued/dormant/halted init projects
Source code is available for download to anyone interested (in reviving them). Some of the projects listed below still compile/work while others may need some tweaking to make them work (again).
| System | Description | Latest release |
|---|---|---|
| cinit (2, 3) | dependency based parallel init without subprocess output logging ala minit/daemontools | 2009-09-29 |
| daemond | daemon control and init daemon written in C++ (akin to dinit) | 2003-11-14 |
| depinit | supports parallel execution, dependencies, true roll-back, log pipelines, improved signaling, and unmounting of filesystems on shutdown | 2003-10-06 |
| einit (2, 3, 4) | modular, parallel, asynchronous, dependency based init system; XML (service) config files | 2007-12-18 |
| ettcl (doc) | custom Tcl-7.6 based interpreter to run the ETLinux init, crond, httpd, and telnetd daemons written in Tcl | 2005-02-08 |
| jinit | C++ init daemon supporting dependency tracking akin to the "need" concept | 2003-04-22 |
| minit | small dependency ordered, parallel init with proper subprocess supervision and logging | 2005-??-?? |
| ninit | dependency based parallel init with supervision, logging, and time-based scheduling akin to crond (HOWTO) | 2010-01-16 |
| pinit (2) | XML configuration, dependency based fast parallel boot, modular design; uses Glib 2.0 and LibXML 2 | 2003-05-01 |
| qinit (2) | Simple and fast exec-chaining init; no config files, services to (re)start are listed in a header file and compiled in. | 2010-12-12 |
| serel (2) | parallel service launcher with synchronisation and integrity-checking (primarily to speed up SysV init boots, akin to startpar) | 2002-08-06 |
| simpleinit(-msb) | dependency based (implements the "need" concept) sequential init without proper supervision (respawn only, no output logging); shipped with util-linux until v2.20; the msb fork is still used in Source Mage Linux | 2007-11-08 |
| System XVI | modular, self-healing, and interface-oriented service manager and init system | 2016-05-18 |
| twsinit | tiny init largely written in x86 assembler | 2003-09-19 |
| upstart | "event based" init that reacts eagerly to incoming "events" to emulate dependency tracking; leaks memory | 2014-09-04 |
| uselessd (2) | forked from systemd v208 | 2015-01-06 |
process supervisors (overview) and service managers
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
| Program | Description | Latest release |
|---|---|---|
| 66 (2) | s6-rc enhancement utils providing extended service management capabilities | 2018-11-20 |
| anopa (2) | service management suite built around s6; can be used as init via exec chaining | 2017-06-20 |
| Circus (doc) | runs, controls and monitors processes and sockets; written in Python; uses ZeroMQ | 2018-06-15 |
| Comar | Python configuration/network/service manager with dbus integration used in Pardus Linux | 2011-10-24 |
| daemontools | collection of tools for managing UNIX services | 2001-07-12 |
| daemontools encore (2) | an enhanced version of daemontools | 2018-10-14 |
| freedt | an independent daemontools reimplementation | 2014-09-03 |
| god (2) | an easily configurable, extensible, monitoring framework written in Ruby | 2014-03-06 |
| 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 | 2018-02-10 |
| monit | process and system monitoring daemon; system status is viewable directly from commandline, or via a native HTTP(S) web server | 2018-05-29 |
| Mudur | Python init/rc script used in Pardus Linux | 2011-10-06 |
| OpenRC (2) | dependency based service management suite; contains a process supervisor and a simple init alternative | 2018-12-21 |
| perp | persistent process (service) supervisor and managment framework for UNIX | 2013-01-11 |
| procer | process supervisor developed for mongrel2 | 2014-03-18 |
| restartd | Debian process-restarting daemon | 2013-01-11 |
| runit | process supervision suite in the line of daemontools; includes a simple init (2) replacement | 2014-08-10 |
| s6 (2) | small process supervision suite for UNIX ala daemontools, runit, and perp; can be used as stage 2 of process #1 via exec chaining | 2018-11-14 |
| s6-rc (2) | service management suite for s6 | 2018-08-14 |
| shepherd | GNU service manager and init daemon written in Guile (formerly known as GNU dmd) | 2018-09-26 |
| slew | s6(-rc) based boot and service script framework written in rc, the Plan 9 shell | 2018-11-23 |
| ssm | simple service management bash scripts for Linux | 2016-03-31 |
| Tokiclover's Supervision Framework (2) | OpenRC-like service management suite for daemontools(-encore), runit, and s6 | 2018-09-01 |
| supervisord (doc) | process control daemon written in Python | 2018-02-15 |
| systemctl-replacement | drop-in reading systemd service descriptions, with docker-oriented init function, written in Python | 2018-09-10 |
SysV init enhancement addons
| System | Description | Latest release |
|---|---|---|
| insserv | can be used with SysV -based init systems; provides dependency-driven system startup (dependencies are specified by LSB headers within init.d scripts) | 2012-11-14 |
| startpar | can be used by the SysV RC boot system executor to allow parallel process system startup | 2014-02-09 |
Safer, supervision-friendly replacements for crond and atd
Simpler, supervision-friendly crond/atd replacements listed below since systemd provides its own job scheduling capabilities in its weird attempt to oust independent solutions and thereby suck otherwise unrelated functionality into the init system to enforce the usage of its own confined straitjacket framework (ninit can also spawn services on specified dates or time intervals if one really wants to use process #1 for that purpose).
| System | Description | Latest release |
|---|---|---|
| bcron (2) | separate tools for different tasks with strictly controlled communication between them; crontab compatible | 2015-08-12 |
| slicd (2) | modular design (separate tools); crontab compatible | 2016-10-30 |
| uschedule | job scheduling suite that uses separate tools for different tasks | 2004-08-?? |
inetd alternatives
Simpler, safer, supervision-friendly, and more reliable replacements for the network super-server daemon inetd.
Listed below since systemd provides similar features to creep into a totally unrelated problem domain and oust the single-purpose daemons specifically designed for that task in order to take over their business and force its own straitjacket APIs and tools upon every single Linux distro and user (finit provides a builtin inetd for those who need such capabilities in process #1).
| System | Description | Latest release |
|---|---|---|
| couriertcpd | advanced UCSPI TCP server daemon, part of the Courier MTA; supports IPv6 and per address/domain access restrictions and concurrency limits | 2018-11-29 |
| iodp | Input/Output Descriptor Protocol (IODP, resembles UCSPI) toolset for IO from/to TCP/UDP/UNIX sockets and fifos | 2000-08-13 |
| ipsvd | UCSPI TCP/UDP server suite with support for access controls, concurrency limits, and SSLv3 | 2008-06-15 |
| onenetd | TCP-only UCSPI server supporting concurrency limits and IPv6 | 2017-01-24 |
| s6-ipcserver | s6 UCSPI server tool for UNIX local-domain sockets | 2018-11-14 |
| s6-networking (2) | small UCSPI networking suite that provides access controls, concurrency limits, IPv6, and TLS/SSL | 2018-08-15 |
| ucspi-tcp | UCSPI toolset for building TCP client-server applications supporting access controls and concurrency limits | 2000-03-18 |
| ucspi-unix (2) | UNIX local-domain socket UCSPI client/server pair | 2015-08-11 |
| xinetd | powerful inetd replacement providing access controls, concurrency limits, extensive logging capabilities, time based restrictions, defence mechanisms against port scans, and more | 2012-05-09 |
udev alternatives for automatic Linux device file creation and management
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.[1]
Although udev can still be compiled for usage without systemd, Lennart Poettering said that they will not polish udevd outside of systemd[2], 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."
This is just another sad example of systemd's annoying habit of sucking otherwise unrelated functionality into the init system and trying to make independent single-purpose daemons obsolete in an impertinent attempt to enforce its sole usage upon every Linux distro and user.
So to be on the safe side, you probably want to use an alternative to udev:
| DevMan | Description | Last release |
|---|---|---|
| eudev (2, 3) | Gentoo's maintained fork of udev | 2018-10-26 |
| mdev (2, 3, 4) | the plug-and-play manager built into BusyBox | 2018-12-30 |
| mdevd (2) | 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 |
| vdev (2) | portable UNIX userspace device-file manager | 2016-08-08 |
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).
See also
- Gentoo wiki :: init system feature comparison table
- Gentoo wiki :: more detailed init system feature comparison table
- A history of modern init systems (1992-2015)
- init systems overview
- init system features and benefits
- Demystifying the init system (init written in Ruby)
- Process supervision: Problem apparently solved
- How process supervision and service management differ (according to Laurent Bercot, author of s6(-rc))
Init-specific articles:
- Understanding SysV init
- The need concept (as implemented in simpleinit)
- Upstart leaks memory (reported on 2018-03-16, still no reaction whatsoever)
- Hints on how to build, install, configure, and use ninit as your default init
- Hints on how to replace your current init with epoch and runit
- Manjaro wiki :: OpenRC (local copy)
- Gentoo wiki :: supported init systems
- Gentoo wiki :: SysV init
- Gentoo wiki :: OpenRC
- Gentoo wiki :: runit
- Gentoo wiki :: s6(-rc)
- guide: Using OpenRC on Arch Linux
- VoidLinux wiki :: runit
- Obarun wiki :: s6-boot
- Replace systemd with busybox + minirc
- How to remove systemd from an Exherbo GNU/Linux installation
- daemontools intro
- An overview of daemontools family supervision suites
- Hints on how to perform socket activation with s6
- s6 init scheme (2): Exec chaining into different "init stages"
- runit init scheme (2, 3): Process #1 starts (and supervises) 3 subprocesses implementing different "init stages"
- Simpler, more reliable, supervision-friendly alternatives to syslogd and klogd: s6 + s6-ipcserver + ucspilogd + s6-log or runit + socklog
