{{Short description|UNIX system component}} {{About|the Unix process||INIT (disambiguation)}} {{Primary sources|date=December 2020}} {{Lowercase title}} [[File:Version 7 UNIX SIMH PDP11 Etc.png|thumb|Version 7 Unix: {{mono|/etc}} listing, showing {{mono|init}} and {{mono|rc}}]] [[File:Version 7 UNIX SIMH PDP11 Etc Rc.png|thumb|Version 7 Unix: contents of an {{mono|/etc/rc}} Bourne shell script]]
In Unix-like 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 if it dies for any reason. Init is typically assigned process identifier 1.
In Unix systems such as System III and System V, the design of init diverged from the functionality provided by the init in Research Unix and its BSD derivatives. Up until the early 2010s,<ref>{{Cite web |date=2018-11-08 |title=Lennart Poettering on systemd's Tumultuous Ascendancy |website=The New Stack |url=https://thenewstack.io/unix-greatest-inspiration-behind-systemd/ |access-date=2024-01-30 |archive-url=https://web.archive.org/web/20181108025744/https://thenewstack.io/unix-greatest-inspiration-behind-systemd/ |archive-date=2018-11-08 |url-status=live}}</ref>{{failed verification|date=February 2024}} 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, most distros have employed a more modern init system, with most employing the init provided by the systemd project. Some distros have elected to create their own SystemV-init compatible system, such as Gentoo Linux's OpenRC and Void Linux's runit. These projects typically include features originally not in SystemV's init, such as multi-threading or interactive init. Most modern init systems are also able to dynamically start, stop and enable services after boot with prompting by the user.
== {{Anchor|STARTUP-SCRIPTS}}Research Unix-style/BSD-style == Research Unix init runs the initialization shell script located at <code>/etc/rc</code>,<ref>{{man|8|init|v7}}</ref> then launches getty on terminals under the control of <code>/etc/ttys</code>.<ref>{{man|5|ttys|v7}}</ref> There are no runlevels; the <code>/etc/rc</code> file determines what programs are run by init. The advantage of this system is that it is simple and easy to edit manually. However, new software added to the system may require changes to existing files that risk producing an unbootable system.
BSD init was, prior to 4.3BSD, the same as Research UNIX's init;<ref>{{man|8|init|4.2BSD}}</ref><ref>{{man|5|ttys|4.2BSD}}</ref> in 4.3BSD, it added support for running a windowing system such as X on graphical terminals under the control of <code>/etc/ttys</code>.<ref>{{man|8|init|4.3BSD}}</ref><ref>{{man|5|ttys|4.3BSD}}</ref> To remove the requirement to edit <code>/etc/rc</code>, BSD variants have long supported a site-specific <code>/etc/rc.local</code> file that is run in a sub-shell near the end of the boot sequence.
A fully modular system was introduced with NetBSD 1.5 and ported to FreeBSD 5.0, OpenBSD 4.9 and successors. This system executes scripts in the <code>/etc/rc.d</code> directory. Unlike System V's script ordering, which is derived from the filename of each script, this system uses explicit dependency tags placed within each script.<ref>{{cite web | url = http://aplawrence.com/Basics/unix-startup-scripts-2.html | title = Unix and Linux startup scripts, Part 2 | author = Andrew Smallshaw | date = 7 December 2009 | access-date = 6 June 2011 | archive-date = 18 December 2009 | archive-url = https://web.archive.org/web/20091218083059/http://aplawrence.com/Basics/unix-startup-scripts-2.html | url-status = live }}</ref> The order in which scripts are executed is determined by the ''rcorder'' utility based on the requirements stated in these tags.
== {{Anchor|SYSV}}SysV-style == [[File:Sysv-rc-conf.png|thumb|{{Mono|sysv-rc-conf}}, a TUI utility that selects which SysV-style init scripts will be run in each runlevel]]
When compared to its predecessors, AT&T's UNIX System III introduced a new style of system startup configuration,<ref>{{cite web|url=http://minnie.tuhs.org/cgi-bin/utree.pl?file=SysIII%2Fusr%2Fsrc%2Fman%2Fman8%2Finit.8|title=init(8)|work=minnie.tuhs.org|access-date=2015-09-12|archive-date=2021-07-27|archive-url=https://web.archive.org/web/20210727193510/https://minnie.tuhs.org/cgi-bin/utree.pl?file=SysIII%2Fusr%2Fsrc%2Fman%2Fman8%2Finit.8|url-status=live}}</ref> which survived (with modifications) into UNIX System V and is therefore called the "SysV-style init".
At any moment, a running System V is in one of the predetermined number of states, called ''runlevels''. At least one runlevel is the normal operating state of the system; typically, other runlevels represent single-user mode (used for repairing a faulty system), system shutdown, and various other states. Switching from one runlevel to another causes a per-runlevel set of scripts to be run, which typically mount filesystems, start or stop daemons, start or stop the X Window System, shutdown the machine, etc.
=== Runlevels === {{Details|Runlevel}}
The runlevels in System V describe certain states of a machine, characterized by the processes and daemons running in each of them. In general, there are seven runlevels, out of which three runlevels are considered "standard", as they are essential to the operation of a system: {{ordered list |item1_value=0 | Turn off |item2_value=1 | Single-user mode (also known as ''S'' or ''s'') |item3_value=6 | Reboot }} Aside from these standard ones, Unix and Unix-like systems treat runlevels somewhat differently. The common denominator, the <code>/etc/inittab</code> file, defines what each configured runlevel does in a given system.
=== Default runlevels === {| class="wikitable floatright sortable" |- ! Operating system ! Default runlevel |- | AIX | 2<ref>{{Cite web |title=System run level |url=https://www.ibm.com/docs/en/aix/7.3.0?topic=startup-system-run-level |archive-url=https://web.archive.org/web/20260326191124/https://www.ibm.com/docs/en/aix/7.3.0?topic=startup-system-run-level |archive-date=2026-03-26 |access-date=2026-03-26 |website=IBM |language=}}</ref> |- | antiX | 5 |- | Gentoo Linux | 3<ref>{{cite web |url=https://wiki.gentoo.org/wiki/Handbook:X86/Working/Initscripts |title=Initscripts |work=Gentoo Linux Documentation |publisher=Gentoo.org |date=2014-12-13 |access-date=2020-12-08 |archive-date=2020-12-03 |archive-url=https://web.archive.org/web/20201203013811/https://wiki.gentoo.org/wiki/Handbook:X86/Working/Initscripts |url-status=live }}</ref> |- | HP-UX | 3 (console/server/multiuser) or 4 (graphical) |- | Slackware Linux | 3 |- | Solaris / illumos | 3<ref>{{cite web |url=https://docs.oracle.com/cd/E23824_01/html/821-1451/hbrunlevels-13026.html |title=Run Levels |publisher=Oracle |work=Oracle Solaris Administration: Common Tasks |access-date=2017-11-14 |archive-date=2016-04-10 |archive-url=https://web.archive.org/web/20160410084238/http://docs.oracle.com/cd/E23824_01/html/821-1451/hbrunlevels-13026.html |url-status=live }}</ref> |- | UNIX System V Releases 3.x, 4.x | 2 |- | UnixWare 7.x | 3 |}
On Linux distributions defaulting to runlevel 5 in the table on the right, runlevel 5 invokes a multiuser graphical environment running the X Window System, usually with a display manager like GDM or KDM. However, the Solaris and illumos operating systems typically reserve runlevel 5 to shut down and automatically power off the machine.
On most systems, all users can check the current runlevel with either the <code>runlevel</code> or <code>who -r</code> command.<ref>{{cite web |url=http://unixhelp.ed.ac.uk/CGI/man-cgi?runlevel+8 |title=UNIX man pages : runlevel (8) |publisher=Unixhelp.ed.ac.uk |date=1997-05-27 |access-date=2014-07-12 |archive-url=https://web.archive.org/web/20140714112953/http://unixhelp.ed.ac.uk/CGI/man-cgi?runlevel+8 |archive-date=2014-07-14 |url-status=dead }}</ref> The root user typically changes the current runlevel by running the <code>telinit</code> or <code>init</code> commands. The <code>/etc/inittab</code> file sets the default runlevel with the <code>:initdefault:</code> entry.
On Unix systems, changing the runlevel is achieved by starting only the missing services (as each level defines only those that are started / stopped).{{Citation needed|date=May 2013}} For example, changing a system from runlevel 3 to 4 might only start the local X server. Going back to runlevel 3, it would be stopped again.
== {{Anchor|S6}}Other implementations == Traditionally, one of the major drawbacks of init is that it starts tasks serially, waiting for each to finish loading before moving on to the next. When startup processes end up Input/output (I/O) blocked, this can result in long delays during boot. Speeding up I/O, e.g. by using SSDs, may shorten the delays but it does not address the root cause.
Various efforts have been made to replace the traditional init daemons to address this and other design problems, including: === General === {{sort-under}} {| class="wikitable sortable sort-under" style="width: auto; text-align: center; font-size: smaller; table-layout: fixed;" |- ! rowspan="2" | Name ! rowspan="2" | Developer ! colspan="2" | Latest release ! rowspan="2" | License ! rowspan="2" | Notes |- ! Version <!-- latest stable release --> ! Date <!-- YYYY-MM-DD / ISO 8601 --> |- | BootScripts | GoboLinux Scripts Contributors<ref name=GoboLinux>{{cite web |url=https://github.com/gobolinux/Scripts |title=gobolinux/Scripts: 🧰 The GoboLinux scripts collection |website=GitHub |access-date=2026-04-25}}</ref> | 016.02 | {{Dts|2017-08-16}}; {{Time ago|2017-08-16}} | {{open source|GPL}} | {{efn|BootScripts in GoboLinux}} |- | busybox-init | Bruce Perens etc.<ref>{{cite web|url=http://www.codepoet.org/andersen/erik/erik.html|title=Erik Andersen's Homepage|access-date=December 23, 2009|archive-date=June 13, 2010|archive-url=https://web.archive.org/web/20100613073438/http://codepoet.org/andersen/erik/erik.html|url-status=live}}</ref><ref>[http://landley.net/code/ Active Projects] {{Webarchive|url=https://web.archive.org/web/20100729022314/http://landley.net/code/ |date=July 29, 2010 }}, Rob Landley's website</ref><ref>[https://busybox.net/~vda/resume/denys_vlasenko.htm Denys Vlasenko] {{Webarchive|url=https://web.archive.org/web/20170905194844/https://busybox.net/~vda/resume/denys_vlasenko.htm |date=September 5, 2017 }}, BusyBox maintainer</ref> | {{wikidata|property|preferred|references|edit|Q620558|P348}} | {{Dts|{{wikidata|qualifier|preferred|single|Q620558|P348|P577}}}}; {{Time ago|{{wikidata|qualifier|preferred|single|Q620558|P348|P577}}}} | {{open source|GPL-2.0-only Since 1.3.0<ref>{{cite web|url=https://www.busybox.net/license.html|title=License|access-date=February 23, 2018|archive-date=February 24, 2018|archive-url=https://web.archive.org/web/20180224052830/https://www.busybox.net/license.html|url-status=live}}</ref><br>GPL-2.0-or-later Until 1.2.2.1<ref>{{cite document|title=BusyBox 1.2.2.1 changelog|publisher=BusyBox|quote=This is the last release of BusyBox under the old 'GPLv2 or later' dual license. Future versions (containing changes after svn 16112) will just be GPLv2 only, without the 'or later'.}}</ref>}} | {{efn|busybox-init, suited to embedded operating systems, used by Alpine Linux (before being replaced with OpenRC), SliTaz 5 (Rolling), Tiny Core Linux, and VMware ESXi, and used by OpenWrt before it was replaced with procd}} |- | dinit | Davin McCall<ref name=dinit/> | {{wikidata|property|preferred|references|edit|Q116697006|P348|P548=Q51930650}} | {{Dts|{{wikidata|qualifier|preferred|single|Q116697006|P348|P548=Q51930650|P577}}}}; {{Time ago|{{wikidata|qualifier|preferred|single|Q116697006|P348|P548=Q51930650|P577}}}} | {{open source|AL2}} | {{efn|dinit, a service manager and init system.<ref name=dinit>{{cite web |url=https://github.com/davmac314/dinit |title=GitHub - davmac314/dinit: Service monitoring / "init" system |website=GitHub |access-date=2021-12-12 |archive-date=2021-12-12 |archive-url=https://web.archive.org/web/20211212163253/https://github.com/davmac314/dinit |url-status=live }}</ref>}} |- | Epoch | Epoch Contributors<ref name=epoch>{{cite web |url=https://github.com/Subsentient/epoch |title=GitHub - Subsentient/epoch: The Epoch Init System |website=GitHub |access-date=2026-04-25}}</ref> | 1.3.0 | {{Dts|2015-06-24}}; {{Time ago|2015-06-24}} | {{open source|Unlicense}} | {{efn|Epoch, a single-threaded Linux init system focused on simplicity and service management<ref>{{cite web |url=http://universe2.us/epoch.html |title=Epoch Init System Homepage |access-date=2014-07-31 |archive-date=2014-08-02 |archive-url=https://web.archive.org/web/20140802142947/http://universe2.us/epoch.html |url-status=live }}</ref>}} |- | finit | Joachim Wiberg, etc.<ref name=finit>{{cite web |url=https://github.com/finit-project/finit |title=GitHub - finit-project/finit: Fast init for Linux. Cookies included |website=GitHub |access-date=2026-04-25 }}</ref> | 4.16<ref name=finit/> | {{Dts|2026-02-27}}; {{Time ago|2026-02-27}} | {{open source|MIT}} | |- | ginitd | S. M. Wood-Mattheusson | | | | {{efn|ginitd, a software package that consists of an init system and a service management system<ref>{{Cite web |last=Wood-Mattheusson |first=S. M. |date=2024-12-16 |title=ginitd: det effektivaste init-systemet |url=https://ylur.me/gershwin/ginitd }}</ref>}} |- | Initng | Initng Contributors<ref name=initng>{{cite web |url=https://github.com/initng/initng |title=initng/initng: InitNG core components |website=GitHub |access-date=2026-04-25}}</ref> | 0.6.10.2 | {{Dts|2007-03-25}}; {{Time ago|2007-03-25}} | {{open source|GPL-3.0}}<ref name=initng/> | {{efn|Initng, a full replacement of init designed to start processes asynchronously}} |- | launchd | Apple Inc. | 10.4 | {{Dts|2005-04-29}}; {{Time ago|2005-04-29}} | {{Proprietary}} (was APSL then AL2) | {{efn|launchd, a replacement for init in Darwin and Darwin-based operating systems such as macOS and iOS starting with Mac OS X v10.4 (it launches SystemStarter to run old-style <nowiki>'rc.local'</nowiki> and SystemStarter processes)}} |- | OpenRC | OpenRC Developers | {{wikidata|property|preferred|references|edit|Q3273885|P348|P548=Q2804309}} | {{Dts|{{wikidata|qualifier|preferred|single|Q3273885|P348|P548=Q2804309|P577}}}}; {{Time ago|{{wikidata|qualifier|preferred|single|Q3273885|P348|P548=Q2804309|P577}}}} | {{open source|BSD-2-Clause}} | {{efn|OpenRC, a process spawner that utilizes system-provided init, while providing process isolation, parallelized startup, and service dependency; used by Alpine Linux, Gentoo and its derivatives, and available as an option in Devuan and Artix Linux.}} |- | procd | Daniel Golle, etc.<ref name=procd>{{cite web |url=https://github.com/openwrt/procd |title=GitHub - openwrt/procd: [MIRROR] OpenWrt service / process manager |website=GitHub |access-date=2026-04-25 }}</ref> | | {{Dts|2026-03-25}}; {{Time ago|2026-03-25}} | {{open source|GPL-2.0-only}} | |- | runit | Gerrit Pape, runit Developers | {{wikidata|property|preferred|references|edit|Q4048599|P348|P548=Q2804309}} | {{Dts|{{wikidata|qualifier|preferred|single|Q4048599|P348|P548=Q2804309|P577}}}}; {{Time ago|{{wikidata|qualifier|preferred|single|Q4048599|P348|P548=Q2804309|P577}}}} | {{open source|BSD-3-Clause}} | {{efn|runit, a cross-platform full replacement for init with parallel starting of services, used by default in Void Linux<ref>{{Cite web |url=http://voidlinux.org/ |title=Void Linux main page |access-date=2020-08-31 |archive-date=2020-08-29 |archive-url=https://web.archive.org/web/20200829154800/https://voidlinux.org/ |url-status=live }}</ref>}} |- | Service Management Facility (SMF) | Sun Microsystems | 5.10 | {{Dts|2005-01-31}}; {{Time ago|2005-01-31}} | {{open source|CDDL}} | {{efn|Service Management Facility (SMF), a complete replacement/redesign of init from the ground up in illumos/Solaris starting with Solaris 10, but launched as the only service by the original System V-style init}} |- | Shepherd | {{wikidata|property|references|edit|Q21194368|P126}}<ref>{{cite web |url=https://openhub.net/p/gnu_shepherd/contributors/3061137008526390 |title=GNU Shepherd Open Source Project on Open Hub: Contributor Ludovic Courtès |website=Open Hub |access-date=2026-04-25}}</ref>etc.<ref>{{cite web |url=https://openhub.net/p/gnu_shepherd/contributors/summary |title=GNU Shepherd Open Source Project on Open Hub: Contributors |website=Open Hub |access-date=2026-04-25}}</ref> | {{wikidata|property|preferred|references|edit|Q21194368|P348|P548=Q55567596}} | {{Dts|{{wikidata|qualifier|preferred|single|Q21194368|P348|P548=Q55567596|P577}}}}; {{Time ago|{{wikidata|qualifier|preferred|single|Q21194368|P348|P548=Q55567596|P577}}}} | {{open source|GPL-3.0-or-later}} | {{efn|Shepherd, the GNU service and daemon manager which provides asynchronous, dependency-based initialisation; written in Guile Scheme and meant to be interactively hackable during normal system operation<ref>{{cite web |url=https://www.gnu.org/software/shepherd/ |title=The Shepherd - GNU Project |publisher=Free Software Foundation, Inc. |access-date=2016-01-16 |archive-date=2016-02-12 |archive-url=https://web.archive.org/web/20160212224150/https://www.gnu.org/software/shepherd/ |url-status=live }}</ref>}} |- | s6 | Laurent Bercot<ref>{{cite web |url=https://github.com/skarnet/s6 |title=skarnet/s6 |website=GitHub |access-date=2026-04-25}}</ref> | {{wikidata|property|preferred|references|edit|Q109312415|P348|P548=Q2804309}} | {{Dts|{{wikidata|qualifier|preferred|single|Q109312415|P348|P548=Q2804309|P577}}}}; {{Time ago|{{wikidata|qualifier|preferred|single|Q109312415|P348|P548=Q2804309|P577}}}} | {{open source|ISC}} | {{efn|s6, a software suite that includes an init system<ref>{{cite web |url=https://www.skarnet.org/software/s6/why.html |title=s6: why another supervision suite |access-date=2021-09-13 |archive-date=2021-09-13 |archive-url=https://web.archive.org/web/20210913201711/https://skarnet.org/software/s6/why.html |url-status=live }}</ref><ref>{{cite web |url=https://www.linuxquestions.org/questions/slackware-14/s6-init-system-4175658553/ |url-status=dead |archive-url=https://web.archive.org/web/20210913201707/https://www.linuxquestions.org/questions/slackware-14/s6-init-system-4175658553/ |archive-date=2021-09-13 |title=s6 init system}}</ref>}} |- | systemd | Red Hat<ref>{{citation | url = https://cgit.freedesktop.org/systemd/systemd/tree/README | website = freedesktop.org | title = systemd README | access-date = 9 September 2012 | archive-date = 7 July 2013 | archive-url = https://web.archive.org/web/20130707193625/http://cgit.freedesktop.org/systemd/systemd/tree/README | url-status = live }}</ref>345 in 2018<ref>{{cite web |title=Systemd Hits A High Point For Number Of New Commits & Contributors During 2018 - Phoronix |url=https://www.phoronix.com/scan.php?page=news_item&px=Systemd-EOY2018-Stats |access-date=30 December 2018 |archive-date=21 September 2009 |archive-url=https://web.archive.org/web/20090921053923/https://www.phoronix.com/scan.php?page=news_item&px=Systemd-EOY2018-Stats |url-status=live }}</ref>2,032 total<ref>Used the "contributors" statistic from: {{Citation |title=systemd/systemd |date=2023-12-03 |url=https://github.com/systemd/systemd |access-date=2023-12-03 |publisher=systemd}}</ref> | {{wikidata|property|preferred|references|edit|Q286124|P348|P548=Q2804309}} | {{Dts|{{wikidata|qualifier|preferred|single|Q286124|P348|P548=Q2804309|P577}}}}; {{Time ago|{{wikidata|qualifier|preferred|single|Q286124|P348|P548=Q2804309|P577}}}} | {{open source|LGPL-2.1-or-later}}<ref name=systemdlicense>{{citation | url = https://0pointer.de/blog/projects/systemd-update-3.html | title = systemd Status Update | last = Poettering | first = Lennart | date = 21 April 2012 | access-date = 28 April 2012 | archive-date = 23 April 2012 | archive-url = https://web.archive.org/web/20120423183615/http://0pointer.de/blog/projects/systemd-update-3.html | url-status = live }}</ref> | {{efn|systemd, a software suite, full replacement for init in Linux that includes an init daemon, with concurrent starting of services, service manager, and other features. Used by Debian (replaces SysV init) and Ubuntu, among other popular Linux distributions. {{As of|2019|2}}, systemd has been adopted by most major Linux distributions.<ref>See Systemd#Adoption</ref>}} |- | SystemStarter | Wilfredo Sanchez | 10.4 | {{Dts|2005-04-29}}; {{Time ago|2005-04-29}} | {{open source|BSD}} | {{efn|SystemStarter, a process spawner started by the BSD-style init in Mac OS X prior to Mac OS X v10.4}} |- | Upstart | Canonical Ltd. | 1.13.2<ref>{{cite web|url=https://launchpad.net/upstart/1.x/1.13.2|title=1.13.2 "It looks lush from the side" : Series 1.x : upstart |publisher=Launchpad|access-date=2022-01-01}}</ref> | {{Dts|2014-09-04}}; {{Time ago|2014-09-04}} | {{open source|GPL-2.0-only}} | {{efn|Upstart, a full replacement of init designed to start processes asynchronously. Initiated by Ubuntu and used by them until 2014. It was also used in Fedora 9,<ref>{{citation |url=https://fedoraproject.org/wiki/Releases/14/FeatureList |title=Fedora 14 Accepted Features |date=2010-07-13 |access-date=2010-07-13 |archive-date=2022-03-27 |archive-url=https://web.archive.org/web/20220327191742/https://fedoraproject.org/wiki/Releases/14/FeatureList |url-status=live }}</ref><ref>{{cite web |url=https://lwn.net/Articles/405100/ |title=Fedora defers systemd to F15 |date=2010-09-14 |access-date=2010-09-17 |publisher=Linux Weekly News |archive-date=2010-09-19 |archive-url=https://web.archive.org/web/20100919142259/http://lwn.net/Articles/405100/ |url-status=live }}</ref> Red Hat Enterprise Linux 6<ref>{{cite web |url=https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/6/html/6.0_technical_notes/deployment |publisher=Red Hat |title=Deployment |work=Red Hat Enterprise Linux 6: Technical Notes |access-date=2013-12-31 |archive-date=2018-08-29 |archive-url=https://web.archive.org/web/20180829175441/https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/6/html/6.0_technical_notes/deployment |url-status=live }}</ref> and Google's ChromeOS.<ref name="chrome-os">{{citation |url=https://sites.google.com/a/chromium.org/dev/chromium-os/chromiumos-design-docs/software-architecture |title=Software Architecture: Chromium OS design documents |access-date=25 January 2014 |archive-date=9 April 2022 |archive-url=https://web.archive.org/web/20220409203000/https://sites.google.com/a/chromium.org/dev/chromium-os/chromiumos-design-docs/software-architecture |url-status=live }}</ref>}} |- class="sortbottom" ! rowspan="2" | Name ! rowspan="2" | Developer ! colspan="2" | Latest release ! rowspan="2" | License ! rowspan="2" | Notes |- ! Version <!-- latest stable release --> ! Date <!-- YYYY-MM-DD / ISO 8601 --> |}
{{reflist|group=lower-alpha}}
=== Operating system support === {| class="wikitable sortable sort-under" style="width: auto; text-align: center; font-size: smaller; table-layout: fixed;" |- ! Name!! Linux!! BSD!! Darwin!! Other |- | BootScripts|| {{Yes}}|| {{No}}|| {{No}}|| {{No}} |- | busybox-init|| {{Yes}}|| {{No}}|| {{No}}|| {{No}} |- | dinit|| {{Yes}}|| {{Yes}}|| {{Yes}}|| {{No}} |- | Epoch|| {{Yes}}|| {{No}}|| {{No}}|| {{No}} |- | finit|| {{Yes}}|| {{No}}|| {{No}}|| {{No}} |- | Initng|| {{Yes}}|| {{No}}|| {{No}}|| {{No}} |- | launchd|| {{No}}|| {{No}}|| {{Yes}}|| {{No}} |- | OpenRC|| {{Yes}}|| {{Yes}}|| {{No}}|| {{No}} |- | procd|| {{Yes}}|| {{No}}|| {{No}}|| {{No}} |- | runit|| {{Yes}}|| {{Yes}}|| {{Yes}}|| {{No}} |- | Service Management Facility (SMF)|| {{No}}|| {{No}}|| {{No}}|| {{Yes|Solaris}} |- | Shepherd|| {{Yes}}|| {{No}}|| {{No}}|| {{Yes|GNU Hurd}} |- | s6|| {{Yes}}|| {{Yes}}|| {{Yes}}|| {{No}} |- | systemd|| {{Yes}}|| {{No}}|| {{No}}|| {{No}} |- | SystemStarter|| {{No}}|| {{No}}|| {{Yes}}|| {{No}} |- | Upstart|| {{Yes}}|| {{No}}|| {{No}}|| {{No}} |- class="sortbottom" ! Name!! Linux!! BSD!! Darwin!! Other |}
=== Compatibility, interface and programming === {| class="wikitable sortable sort-under" style="width: auto; text-align: center; font-size: smaller; table-layout: fixed;" |+ Comparison of init systems<ref name=systemdfree>{{Cite web |url=https://systemdfree.com/ |title=systemdfree.com — Because your init should do one thing |access-date=2026-04-25 |quote=As of 2024, the systemd project encompasses: an init daemon, a service manager, a login manager (logind), a DNS resolver (systemd-resolved), a time synchronization daemon (systemd-timesyncd), a network manager (systemd-networkd), a bootloader (systemd-boot), a home directory manager (systemd-homed), a credential manager, a container manager (systemd-nspawn), a journal binary log system (journald), a device manager (udev, absorbed in 2012), and a cryptography layer for encrypted storage. These are not plugins. They are deeply coupled components sharing internal APIs, dbus interfaces, and cgroup hierarchies. [...] GNOME's dependency on logind, NetworkManager's integration with systemd-networkd, and PulseAudio's replacement by PipeWire — which also integrates deeply with systemd session management}}</ref><ref>{{Cite web |url=https://wiki.gentoo.org/wiki/Comparison_of_init_systems |title=Comparison of init systems - Gentoo wiki |date=2025-09-16 |access-date=2026-04-25}}</ref> |- ! Name ! musl<br>libc<br>compatible ! dependency<br>free ! script/<br>service<br>format ! Plain<br>log<br>format ! Per-<br>service<br>config ! Cross-<br>service<br>events ! Parallel<br>service<br>startup ! Process<br>supervision ! Programming language ! Codebase<br>size<br>(lines) |- | BootScripts | {{?}} | {{?}} | {{Yes|Shell<br>scripts}}<ref name=GoboLinux/> | {{?}} | {{?}} | {{?}} | {{?}} | {{?}} | {{Bad|Shell 66.6%; C 19.0%; Python 12.7%; Other 1.7%}}<ref name=GoboLinux/> |- | busybox-init | {{?}} | {{?}} | {{?}} | {{?}} | {{?}} | {{?}} | {{?}} | {{?}} | {{Yes|C 90.4%; shell script 4.6%; C++ 1.7%; HTML 1.4%; Assembly 0.8%; Make 0.5%; XML 0.2%; Perl 0.3%; Python 0.1%}}<ref name=BusyBoxLang>{{cite web |url=https://openhub.net/p/busybox/analyses/latest/languages_summary |title=The BusyBox Open Source Project on Open Hub: Languages Page |website=Open Hub |access-date=2026-04-25}}</ref> | {{No|{{Nts|319897}}}}<br><ref name=BusyBoxLang/> |- | dinit | {{Yes}} | {{Yes}} | {{Yes|Text<br>config}} | {{Yes}} | {{Yes}} | {{Yes}} | {{Yes}} | {{Yes|Native}} | {{Yes|C++ 96.1%; Starlark 1.0%; C 1.0%; Shell 0.9%; Makefile 0.7%; Zig 0.2%; Go 0.1%}}<ref name=dinit/> | {{Yes|{{Nts|25000|prefix=~}}}} |- | Epoch | {{Yes}} | {{Yes|libc,<br>/bin/sh}} | {{Yes|multiple<br>or single<br>.conf}} | {{Yes}} | {{Yes}}<br>(v1.1+) | {{Yes|(basic<br>support,<br>v1.3+)}} | {{No}} | {{Yes}} | {{Yes|C 98.2%; shell script 1.8%; Make 0.1%}}<ref name=EpochLang>{{cite web |url=https://openhub.net/p/epoch/analyses/latest/languages_summary |title=The Epoch Init System Open Source Project on Open Hub: Languages Page |website=Open Hub |access-date=2026-04-25}}</ref> | {{Yes|{{Nts|10546}}}}<ref name=EpochLang/> |- | finit | {{?}} | {{?}} | {{Yes|multiple<br>or single<br>.conf}} | {{Yes}} | {{?}} | {{?}} | {{Yes}} | {{Yes}} | {{Yes|C 86.7%; shell script 9.7%; Automake 1.8%; Autoconf 1.5%; Make 0.3%}}<ref name=finitLang>{{cite web |url=https://openhub.net/p/finit/analyses/latest/languages_summary |title=The finit Open Source Project on Open Hub: Languages Page |website=Open Hub |access-date=2026-04-25}}</ref> | {{Yes|{{Nts|33034}}}}<ref name=finitLang/> |- | Initng | {{?}} | {{?}} | {{?}} | {{?}} | {{?}} | {{?}} | {{?}} | {{?}} | {{Partial|C 67.5%; Assembly 12.8%; shell script 13.2%; Python 2.7%; CMake 1.5%; Jam 1.1%; HTML 0.6%; Make 0.5%; Vim Script 0.1%}}<ref name=InitngLang>{{cite web |url=https://openhub.net/p/initng/analyses/latest/languages_summary |title=The InitNG Open Source Project on Open Hub: Languages Page |website=Open Hub |access-date=2026-04-25}}</ref> | {{Partial|{{Nts|59471}}}}<ref name=InitngLang/> |- | launchd | {{?}} | {{Yes}} | {{Yes|plist}} | {{?}} | {{?}} | {{No}} | {{Yes}} | {{Yes}} | {{Yes|C 96.1%; shell script 2.2%; C++ 1.3%; Make 0.4%; XML 0.1%}}<ref name=launchdLang>{{cite web |url=https://openhub.net/p/openlaunchd/analyses/latest/languages_summary |title=The Open launchd Open Source Project on Open Hub: Languages Page |website=Open Hub |access-date=2026-04-25}}</ref> | {{Yes|{{Nts|28128}}}}<ref name=launchdLang/> |- | OpenRC | {{Yes}} | {{Yes|init}}<br>(sysv or BSD) | {{Yes|Shell<br>scripts}} | {{Yes}} | {{Yes}}<br>(conf.d) | {{Yes}} | {{Yes|Disabled<br>by<br>default}} | {{Partial|Via<br>external<br>tool}} | {{Yes|C 87.6%; shell script 12.2%; Perl 0.2%}}<ref name=OpenRCLang>{{cite web |url=https://openhub.net/p/open_rc/analyses/latest/languages_summary |title=The OpenRC Open Source Project on Open Hub: Languages Page |website=Open Hub |access-date=2026-04-25}}</ref> | {{Yes|{{Nts|23827}}}}<ref name=OpenRCLang/> |- | procd | {{?}} | {{?}} | {{Yes|Shell<br>scripts}}<ref name=procdConfig>{{Cite web |url=https://openwrt.org/docs/guide-user/base-system/uci |title=[OpenWrt Wiki] The UCI system |author=stokito |date=2025-11-18 |access-date=2026-04-25}}</ref> | {{?}} | {{Yes}}<ref name=procdConfig/> | {{?}} | {{?}} | {{Yes}}<ref name=procdTechref>{{Cite web |url=https://openwrt.org/docs/techref/procd |title=[OpenWrt Wiki] Procd system init and daemon management |author=bluewavenet |date=2025-02-18 |access-date=2026-04-25}}</ref> | {{Yes|C 98.8%; CMake 0.8%; shell script 0.4%}}<ref name=procdLang>{{cite web |url=https://openhub.net/p/openwrt-procd/analyses/latest/languages_summary |title=The openwrt-procd Open Source Project on Open Hub: Languages Page |website=Open Hub |access-date=2026-04-25}}</ref> | {{Yes|{{Nts|18832}}}}<ref name=procdLang/> |- | runit | {{Yes}} | {{Yes}} | {{Yes|Shell<br>scripts}} | {{Yes}} | {{No}} | {{Yes}} | {{Partial|Via<br>supervision<br>trees}} | {{Yes|Native}} | {{Partial|C 57.4%; HTML 32.1%; shell script 6.3%; Make 4.0%; XML 0.2%}}<ref name=runitLang>{{cite web |url=https://openhub.net/p/runit/analyses/latest/languages_summary |title=The runit Open Source Project on Open Hub: Languages Page |website=Open Hub |access-date=2026-04-25}}</ref> | {{Yes|{{Nts|11616}}}}<ref name=runitLang/> |- | Service Management Facility (SMF) | {{?}} | {{Yes|init}}<br>(sysv?) | {{Yes|XML<br>(+ shell<br>scripts)}} | {{Yes}} | {{Yes}}<br>(service<br>instances) | {{Yes}} | {{Yes}} | {{Yes}} | {{Yes|C}} |- | Shepherd | {{?}} | {{?}} | {{?}} | {{?}} | {{?}} | {{?}} | {{?}} | {{?}} | {{Bad|Scheme 70.5%; shell script 28.3%; Automake 0.7%; Autoconf 0.4%}}<ref name=ShepherdLang>{{cite web |url=https://openhub.net/p/gnu_shepherd/analyses/latest/languages_summary |title=The GNU Shepherd Open Source Project on Open Hub: Languages Page |website=Open Hub |access-date=2026-04-25}}</ref> | {{Yes|{{Nts|6606}}}}<ref name=ShepherdLang/> |- | s6 | {{Yes}} | {{Yes|execline}} | {{Yes|execline/<br>shell}} | {{Yes}} | {{No}} | {{Yes}} | {{Yes}} | {{Yes|Native}} | {{Bad|C 48.3%; HTML 46.1%; shell script 2.8%; Autoconf 2.0%; Make 0.7%; Emacs Lisp 0.1%}}<ref name=s6Lang>{{cite web |url=https://openhub.net/p/s6/analyses/latest/languages_summary |title=The s6 Open Source Project on Open Hub: Languages Page |website=Open Hub |access-date=2026-04-25}}</ref> | {{Yes|{{Nts|31069}}}}<ref name=s6Lang/> |- | systemd | {{No|Limited}} | {{No|dbus udev dns ntp GNOME NetworkManager PipeWire ...<ref name=systemdfree/>}} | {{No|Binary-<br>compiled<br>units}} | {{No}}<br>(journald) | {{Yes}} | {{Yes}} | {{Yes}} | {{Partial|Built-in<br>but<br>opaque}} | {{Partial|C 77.9%; XML 12.5%; shell script 3.9%; Python 3.2%; C++ 2.4%; CSS 0.1%}}<ref name=systemdLang>{{cite web |url=https://openhub.net/p/systemd/analyses/latest/languages_summary |title=The systemd Open Source Project on Open Hub: Languages Page |website=Open Hub |access-date=2026-04-25}}</ref> | {{No|{{Nts|1310214}}}}<br><ref name=systemdLang/> |- | Upstart | {{?}} | {{?}} | {{?}} | {{?}} | {{?}} | {{?}} | {{?}} | {{?}} | {{Yes|C 93.1%; Python 2.5%; Automake 1.1%; C++ 2.4%; shell script 0.3%; XML 0.3%; Autoconf 0.1%; Vim Script 0.1%}}<ref name=UpstartLang>{{cite web |url=https://openhub.net/p/upstart/analyses/latest/languages_summary |title=The Upstart Open Source Project on Open Hub: Languages Page |website=Open Hub |access-date=2026-04-25}}</ref> | {{Partial|{{Nts|126865}}}}<br><ref name=UpstartLang/> |- class="sortbottom" ! Name ! musl<br>libc<br>compatible ! dependency<br>free ! script/<br>service<br>format ! Plain<br>log<br>format ! Per-<br>service<br>config ! Cross-<br>service<br>events ! Parallel<br>service<br>startup ! Process<br>supervision ! Programming language ! Codebase<br>size<br>(lines) |}
== See also == * Operating system service management * Session Manager Subsystem — an equivalent in Windows NT
== References == {{Reflist|30em}}
== External links == * [https://www.freebsd.org/cgi/man.cgi?query=init&sektion=8&format=html FreeBSD init man page] * [https://arxiv.org/abs/0706.2748 A paper summarizing Unix init schemes] (2007) * {{webarchive |url=https://web.archive.org/web/20051231110711/http://www.sun.com/bigadmin/content/selfheal/smf-quickstart.html |date=December 31, 2005 |title=Solaris Service Management Facility - Quickstart Guide }} * [http://blog.darknedgy.net/technology/2015/09/05/0/ A history of modern init systems (1992–2015)] {{Webarchive|url=https://web.archive.org/web/20151011235646/http://blog.darknedgy.net/technology/2015/09/05/0/ |date=2015-10-11 }}
{{Service management in Unix}}
Category:Unix process- and task-management-related software