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In Unix-based computer operating systems, init short for initialization 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 2015update, systemd has been adopted by several major Linux distributions although it remains controversial
- 1 Research Unix-style/BSD-style
- 2 SysV-style
- 21 Runlevels
- 22 Default runlevels
- 3 Replacements for init
- 4 See also
- 5 References
- 6 External links
Research Unix init ran the initialization shell script located in /etc/rc,1 then launched getty on terminals under the control of /etc/ttys2 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 43BSD, the same as Research UNIX's init;34 in 43BSD, it added support for running a windowing system such as X on graphical terminals under the control of /etc/ttys56 To remove the requirement to edit /etc/rc, BSD variants have long supported a site-specific /etc/rclocal file that is run in a sub-shell near the end of the boot sequence
A fully modular system was introduced with NetBSD 15 and ported to FreeBSD 50 and successors This system executes scripts in the /etc/rcd 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 script7 The order in which scripts are executed is determined by the rcorder script based on the requirements stated in these tags
SysV-styleeditSysv-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,8 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
RunlevelseditFor more details on this topic, see 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 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 also known as S or s 6 Reboot
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
|Operating system||Default runlevel|
|HP-UX||3 console/server/multiuser or 4 graphical|
|UNIX System V Releases 3x, 4x||2|
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 command11 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 / stoppedcitation 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
Replacements for initedit
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 Speeding up I/O, eg 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:
- BootScripts in GoboLinux
- busybox-init, suited to 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 desktop environment startedcitation needed
- eINIT, a full replacement of init designed to start processes asynchronously, but with the potential of doing it without shell scripts12
- Epoch, a single-threaded Linux init system focused on simplicity and service management13
- Initng, a full replacement of init designed to start processes asynchronously
- launchd, a replacement for init in Darwin/macOS/iOS/TvOS starting with Mac OS X v104 it launches SystemStarter to run old-style 'rclocal' and SystemStarter processes
- Mudur, an init replacement written in Python and designed to start process asynchronously in use by the Pardus Linux distribution14
- nosh, a suite of system-level utilities for initializing and running a BSD or Linux system, for managing daemons, terminals and logging15
- 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, used by default in Void Linux
- s6, another cross-platform full replacement for init, similar to runit16
- Service Management Facility, 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, 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 operation17
- 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 v104
- Upstart, a full replacement of init designed to start processes asynchronously initiated by Ubuntu
- Operating system service management
- smssexe — an equivalent in Windows NT
- ^ init8 – Version 7 Unix Programmer's Manual
- ^ ttys5 – Version 7 Unix Programmer's Manual
- ^ init8 – 42BSD System Manager's Manual
- ^ ttys5 – 42BSD File Formats Manual
- ^ init8 – 43BSD System Manager's Manual
- ^ ttys5 – 43BSD File Formats Manual
- ^ Andrew Smallshaw 7 December 2009 "Unix and Linux startup scripts, Part 2"
- ^ "init8" minnietuhsorg
- ^ "Initscripts" Gentoo Linux Documentation Gentooorg 2011-03-02 Retrieved 2011-06-13
- ^ "Oracle Documentation" Docssuncom 2010-09-07 Retrieved 2011-06-13
- ^ "UNIX man pages : runlevel 8" Unixhelpedacuk 1997-05-27 Retrieved 2014-07-12
- ^ "eINIT git repository XML module directory"
- ^ "Epoch Init System Homepage"
- ^ Gürer Özen, Görkem Çetin "Speeding Up Linux: One Step Further With Pardus Pardus" Pardusorgtr Retrieved 2011-06-13
- ^ Jonathan de Boyne Pollard "The nosh package" Jonathan de Boyne Pollard Retrieved 2015-08-02
- ^ "s6 - skarnet's small supervision suite"
- ^ "The Shepherd - GNU Project" Free Software Foundation, Inc Retrieved 2016-01-16
- 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
|Process supervision tools||
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