init

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In Unix-like computer operating systems, init (short for initialization) is the first user-space 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; in most Unix-like systems, 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,^{[1][failed verification]} 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 Linux distributions have employed a more modern init system, with most employing the init provided by the systemd project. Some distributions have elected to create their own System V-init compatible system, such as Gentoo Linux's OpenRC and Void Linux's runit. These projects typically include features originally not in System V'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.

Research Unix-style/BSD-style

Research Unix init runs the initialization shell script located at /etc/rc,^[2] then launches getty on terminals under the control of /etc/ttys.^[3] There are no runlevels; the /etc/rc 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;^[4][5] in 4.3BSD, it added support for running a windowing system such as X on graphical terminals under the control of /etc/ttys.^[6][7] To remove the requirement to edit /etc/rc, BSD variants have long supported a site-specific /etc/rc.local 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 /etc/rc.d 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.^[8] The order in which scripts are executed is determined by the rcorder utility based on the requirements stated in these tags.

SysV-style

When compared to its predecessors, AT&T's UNIX System III introduced a new style of system startup configuration,^[9] 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.

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

Name	Developer	Latest release		License	Notes
		Version	Date
BootScripts	GoboLinux Scripts Contributors[14]	016.02	August 16, 2017; 9 years ago	GPL	[a]
busybox-init	Bruce Perens etc.[15][16][17]	1.36.1[18]	19 May 2023; 3 years ago	GPL-2.0-only Since 1.3.0[19] GPL-2.0-or-later Until 1.2.2.1[20]	[b]
dinit	Davin McCall[21]	0.22.1[22]	4 July 2026; 2 months ago	AL2	[c]
Epoch	Epoch Contributors[23]	1.3.0	June 24, 2015; 11 years ago	Unlicense	[d]
finit	Joachim Wiberg, etc.[25]	4.17[25]	April 28, 2026; 4 months ago	MIT
ginitd	S. M. Wood-Mattheusson[26]				[e]
Initng	Initng Contributors[27]	0.6.10.2	March 25, 2007; 19 years ago	GPL-3.0[27]	[f]
launchd	Apple Inc.	10.4	April 29, 2005; 21 years ago	Proprietary (was APSL then AL2)	[g]
OpenRC	Roy Marples[28] etc.[29]	0.63.3[30]	2 July 2026; 2 months ago	BSD-2-Clause	[h]
procd	Daniel Golle, etc.[31]		June 17, 2026; 2 months ago	GPL-2.0-only
runit	Gerrit Pape[32] etc.[33]	2.3.1[34]	7 February 2026; 7 months ago	BSD-3-Clause	[i]
s6	Laurent Bercot[36]	2.15.1.0[37]	14 July 2026; 56 days ago	ISC	[j]
Service Management Facility (SMF)	Sun Microsystems	5.10	January 31, 2005; 21 years ago	CDDL	[k]
Shepherd	Ludovic Courtès [40]etc.[41]	1.0.9[42]	3 December 2025; 9 months ago	GPL-3.0-or-later	[l]
systemd	Red Hat[44]345 in 2018[45]2,032 total[46]	261.2[47]	23 July 2026; 47 days ago	LGPL-2.1-or-later[48]	[m]
SystemStarter	Wilfredo Sanchez[50]	10.4	April 29, 2005; 21 years ago	BSD	[n]
Upstart	Canonical Ltd.	1.13.2[51]	September 4, 2014; 12 years ago	GPL-2.0-only	[o]
Name	Developer	Latest release		License	Notes
		Version	Date

1. ↑ BootScripts in GoboLinux
2. ↑ 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
3. ↑ dinit, a service manager and init system.^[21]
4. ↑ Epoch, a single-threaded Linux init system focused on simplicity and service management^[24]
5. ↑ ginitd, a software package that consists of an init system and a service management system^[26]
6. ↑ Initng, a full replacement of init designed to start processes asynchronously
7. ↑ 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 'rc.local' and SystemStarter processes)
8. ↑ 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.
9. ↑ runit, a cross-platform full replacement for init with parallel starting of services, used by default in Void Linux^[35]
10. ↑ s6, a software suite that includes an init system^[38][39]
11. ↑ 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
12. ↑ 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^[43]
13. ↑ 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 February 2019^[update], systemd has been adopted by most major Linux distributions.^[49]
14. ↑ SystemStarter, a process spawner started by the BSD-style init in Mac OS X prior to Mac OS X v10.4
15. ↑ 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,^[52][53] Red Hat Enterprise Linux 6^[54] and Google's ChromeOS.^[55]

See also

• Operating system service management
• Session Manager Subsystem — an equivalent in Windows NT

References

External links

• FreeBSD init man page
• A paper summarizing Unix init schemes (2007)
• Solaris Service Management Facility - Quickstart Guide at the Wayback Machine (archived December 31, 2005)
• A history of modern init systems (1992–2015) Archived 2015-10-11 at the Wayback Machine