init - Wikipedia

UNIX system component

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 has 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 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.

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.

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.

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:

Aside from these standard ones, Unix and Unix-like systems treat runlevels somewhat differently. The common denominator, the /etc/inittab file, defines what each configured runlevel does in a given system.

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 runlevel or who -r command.^[13] The root user typically changes the current runlevel by running the telinit or init commands. The /etc/inittab file sets the default runlevel with the :initdefault: 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]} 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.

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:

Name	Developer	Latest release		License	Notes
		Version	Date
BootScripts	GoboLinux Scripts Contributors[14]	016.02	August 16, 2017; 8 years ago	GPL	[a]
busybox-init	Bruce Perens etc.[15][16][17]	1.36.1[18]	19 May 2023; 2 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]	0.21.0[22]	9 March 2026; 48 days ago	AL2	[c]
Epoch	Epoch Contributors[23]	1.3.0	June 24, 2015; 10 years ago	Unlicense	[d]
finit	Joachim Wiberg, etc.[25]	4.16[25]	February 27, 2026; 58 days ago	MIT
ginitd	S. M. Wood-Mattheusson				[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; 20 years ago	Proprietary (was APSL then AL2)	[g]
OpenRC	OpenRC Developers	0.63.1[28]	3 April 2026; 23 days ago	BSD-2-Clause	[h]
procd	Daniel Golle, etc.[29]		March 25, 2026; 32 days ago	GPL-2.0-only
runit	Gerrit Pape, runit Developers	2.3.1[30]	7 February 2026; 2 months ago	BSD-3-Clause	[i]
Service Management Facility (SMF)	Sun Microsystems	5.10	January 31, 2005; 21 years ago	CDDL	[j]
Shepherd	Ludovic Courtès [32]etc.[33]	1.0.9[34]	3 December 2025; 4 months ago	GPL-3.0-or-later	[k]
s6	Laurent Bercot[36]	2.14.0.1[37]	24 January 2026; 3 months ago	ISC	[l]
systemd	Red Hat[40]345 in 2018[41]2,032 total[42]	260.1[43]	23 March 2026; 34 days ago	LGPL-2.1-or-later[44]	[m]
SystemStarter	Wilfredo Sanchez	10.4	April 29, 2005; 20 years ago	BSD	[n]
Upstart	Canonical Ltd.	1.13.2[46]	September 4, 2014; 11 years ago	GPL-2.0-only	[o]
Name	Developer	Latest release		License	Notes
		Version	Date

Comparison of init systems^[51][52]

Name	musl libc compatible	dependency free	script/ service format	Plain log format	Per-service configuration	Cross- service events	Parallel service startup	Process supervision	Programming language	Codebase size (lines)
BootScripts	?	?	Shell scripts[14]	?	?	?	?	?	Shell 66.6%; C 19.0%; Python 12.7%; Other 1.7%[14]
busybox-init	?	?	?	?	?	?	?	?	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%[53]	319,897 [53]
dinit	Yes	Yes	Text config	Yes	Yes	Yes	Yes	Native	C++ 96.1%; Starlark 1.0%; C 1.0%; Shell 0.9%; Makefile 0.7%; Zig 0.2%; Go 0.1%[21]	~25,000
Epoch	Yes	libc, /bin/sh	multiple or single .conf	Yes	Yes (v1.1+)	(basic support, v1.3+)	No	Yes	C 98.2%; shell script 1.8%; Make 0.1%[54]	10,546[54]
finit	?	?	multiple or single .conf	Yes	?	?	Yes	Yes	C 86.7%; shell script 9.7%; Automake 1.8%; Autoconf 1.5%; Make 0.3%[55]	33,034[55]
Initng	?	?	?	?	?	?	?	?	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%[56]	59,471[56]
launchd	?	Yes	plist	?	?	No	Yes	Yes	C 96.1%; shell script 2.2%; C++ 1.3%; Make 0.4%; XML 0.1%[57]	28,128[57]
OpenRC	Yes	init (sysv or BSD)	Shell scripts	Yes	Yes (conf.d)	Yes	Disabled by default	Via external tool	C 87.6%; shell script 12.2%; Perl 0.2%[58]	23,827[58]
procd	?	?	Shell scripts[59]	?	Yes[59]	?	?	Yes[60]	C 98.8%; CMake 0.8%; shell script 0.4%[61]	18,832[61]
runit	Yes	Yes	Shell scripts	Yes	No	Yes	Via supervision trees	Native	C 57.4%; HTML 32.1%; shell script 6.3%; Make 4.0%; XML 0.2%[62]	11,616[62]
Service Management Facility (SMF)	?	init (sysv?)	XML (+ shell scripts)	Yes	Yes (service instances)	Yes	Yes	Yes	C
Shepherd	?	?	?	?	?	?	?	?	Scheme 70.5%; shell script 28.3%; Automake 0.7%; Autoconf 0.4%[63]	6,606[63]
s6	Yes	execline	execline/ shell	Yes	No	Yes	Yes	Native	C 48.3%; HTML 46.1%; shell script 2.8%; Autoconf 2.0%; Make 0.7%; Emacs Lisp 0.1%[64]	31,069[64]
systemd	Limited	D-Bus	Binary- compiled units	No (journald)	Yes	Yes	Yes	Built-in but opaque	C 77.9%; XML 12.5%; shell script 3.9%; Python 3.2%; C++ 2.4%; CSS 0.1%[65]	1,310,214 [65]
Upstart	?	?	?	?	?	?	?	?	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%[66]	126,865 [66]
Name	musl libc compatible	dependency free	script/ service format	Plain log format	Per-service configuration	Cross- service events	Parallel service startup	Process supervision	Programming language	Codebase size (lines)