In Unix-based computer operating systems, init is the first process
started during booting of the computer 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 using a hard-coded filename; a kernel
panic will occur if the kernel is unable to start it. 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. The usage in most Linux
distributions employing a traditional init rather than a recent variant such
as systemd is somewhat compatible with System V, while some distributions such
as Slackware use BSD-style startup scripts, and others such as Gentoo have
their own customized versions. Several replacement init implementations
have been created, attempting to address design limitations in the standard
versions. These include launchd, the Service Management Facility, systemd and
Upstart; as of March 2015, systemd has been adopted by major Linux
distributions although it remains controversial.
Research Unix-style/BSD-style Research Unix init ran the
initialization shell script located in rc, then launched getty on terminals
under the control of ttys. There are no runlevels; the 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; in 4.3BSD, it
added support for running a windowing system such as X on graphical terminals
under the control of ttys. To remove the requirement to edit rc, BSD variants
have long supported a site-specific 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 and successors. This system executes
scripts in the 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. The order in which scripts are
executed is determined by the rcorder script 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, which survived 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, 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= 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 eight runlevels, out
of which three runlevels are considered "standard" as they are essential to the
operation of a system: 0. Halt
1. Single user mode 6. Reboot
Aside from these standard ones, Unix and Unix-like systems treat runlevels
somewhat differently. The common denominator, the inittab file, defines
what each configured runlevel does in a given system.
= Default runlevels= 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 operating system typically reserves 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. The root typically changes the current runlevel
by running the telinit or init commands. The inittab file sets the default
runlevel with the :initdefault: entry. On Unix systems, changing the runlevel
is achieved by starting only the missing services. 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. Replacements for init
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 I/O blocked, this can result in long delays
during boot. Various efforts have been made to
replace the traditional init daemons to address this and other design problems,
including: BootScripts in GoboLinux
busybox-init, suited embedded operating systems, employed by OpenWrt before it
was replaced with procd DEMONS, a modification of the init start
process by KahelOS, where daemons are started only when the DE started
eINIT, a full replacement of init designed to start processes
asynchronously, but with the potential of doing it without shell scripts
dmd, a GNU project for a service manager acting as a dependency-based init
replacement Epoch, a single-threaded Linux init
system focused on simplicity and service management
Initng, a full replacement of init designed to start processes
asynchronously launchd, a replacement for init
introduced in Mac OS X v10.4 Mudur, an init replacement written in
Python and designed to start process asynchronously in use by the Pardus
Linux distribution nosh, a suite of system-level utilities
for initializing and running a BSD or Linux system, for managing daemons,
terminals and logging OpenRC, a process spawner that utilizes
system-provided init, while providing process isolation, parallelized startup,
and service dependency; used by Gentoo and its derivatives
runit, a cross-platform full replacement for init with parallel starting of
services s6, another cross-platform full
replacement for init, similar to runit Service Management Facility, a complete
replacement/redesign of init from the ground up in Solaris starting with
Solaris 10, but launched as the only service by the original System V-style
init systemd, a full replacement for init in
Linux with concurrent starting of services and other features, used by
influential Linux distributions SystemStarter, a process spawner started
by the BSD-style init in OS X prior to Mac OS X v10.4
Upstart, a full replacement of init designed to start processes
asynchronously initiated by Ubuntu. See also
Operating system service management References
External links FreeBSD init man page
Paper summarizing Unix init schemes smf and milestones in Solaris 10