In Unix-based computer operating systems, init (short for initialization) is a daemon process that is the direct or indirect ancestor of all other processes. It automatically adopts all orphaned processes. Init is the first process started during booting, and is typically assigned PID number 1. It is started by the kernel using a hard-coded filename, and if the kernel is unable to start it, a kernel panic will result. Init continues running until the system is shut down.
The design of init has diverged in Unix systems such as System III and System V, from the functionality provided by the init in Research Unix and its BSD derivatives. The usage on most Linux distributions is somewhat compatible with System V, but some distributions, such as Slackware, use a BSD-style and others, such as Gentoo, have their own customized version.
Several replacement init implementations have been written with attempt to address design limitations in the standard versions. These include systemd and Upstart, the latter being used by Ubuntu and some other Linux distributions.
System V init examines the
/etc/inittab file for an
:initdefault: entry, which defines any default runlevel. If there is no default runlevel, then
init dumps the user to a system console for manual entry of a runlevel.
The runlevels in System V describe certain states of a machine, characterized by the processes run. There are generally 8 runlevels. Of these eight, 3 are so-called "reserved" runlevels: these are the runlevels 0 to 6 and S or s, which are aliased to the same runlevel.
Aside from runlevels 0, 1, and 6, every Unix and Unix-like system treats runlevels a little differently. The common denominator, the
/etc/inittab file, defines what each runlevel does (if they do anything at all) in a given system.
Default runlevels 
|Operating System||Default runlevel|
|CentOS||3 (console/server) or 5 (graphical/desktop)|
|HP-UX||3 (console/server/mulituser) or 4 (graphical)|
|Mac OS X||3|
|Mandriva Linux||3 (console/server) or 5 (graphical/desktop)|
|Red Hat Enterprise Linux / Fedora||3 (console/server) or 5 (graphical/desktop)|
|SUSE Linux Enterprise/openSUSE Linux||3 (console/server) or 5 (graphical/desktop)|
|Ubuntu (Server and Desktop)||2|
On the Linux distributions defaulting to runlevel 5 in the table above, runlevel 5 invokes a multiuser graphical environment running the X Window System, usually with a display manager. However, the Solaris operating system typically reserves runlevel 5 to shut down and automatically power off the machine. Other OS or playful users might also define additional runlevels.
On most systems users can check the current runlevel with either of the following commands:
$ who -r
The root typically changes the current runlevel by running the
init commands. The
/etc/inittab file sets the default runlevel with the
On Unix systems, entering or leaving a runlevel for another one is achieved by starting only the missing services (as each level defines only those that are added / stopped). I.e. booting a HP-UX system from runlevel 3 to 4 might only start the local X server. Going back to runlevel 3, it would be stopped again.
On some Linux distributions (RHEL5 most notably) each runlevel has the full set of definitions, meaning some unneeded work during transitions. Temporary files in a special directory (/var/lock/subsys) are used to track which software has been launched. Only those programs' stop scripts will be executed, which means that non-compliant software might not be stopped at all.
This is typically observed as multi-user server processes like a database still running after switching to single user mode.
BSD init runs the initialization shell script located in
/etc/rc, then launches getty on text-based terminals or a windowing system such as X on graphical terminals under the control of
/etc/ttys. 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. To mitigate against this, 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 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. The order in which scripts are executed is determined by the rcorder script based on the requirements stated in these tags.
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
- DEMONS, a modification of the init start process by KahelOS, where daemons are started only when the DE (desktop environment) started.
- eINIT, a full replacement of init designed to start processes asynchronously, but with the potential of doing it without shell-scripts
- Initng, a full replacement of init designed to start processes asynchronously
- Initscripts, System initialization/bootup scripts for Arch Linux
- launchd, a replacement for init introduced in Mac OS X v10.4 (it launches SystemStarter to run old-style 'rc.local' and SystemStarter processes)
- Mudur, an init replacement written in Python and designed to start process asynchronously in use by the Pardus Linux distribution.
- 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 full replacement/redesign of init from the ground up in Solaris starting with Solaris 10
- systemd, a full replacement for init with parallel starting of services, reduced shell overhead and other features, used by many distributions.
- SystemStarter, a process spawner started by the BSD-style init in Mac OS X prior to Mac OS X v10.4
- Upstart, a full replacement of init designed to start processes asynchronously initiated by Ubuntu.
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