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InUnix-based computeroperating systems,init (short forinitialization) is the firstprocess started duringbooting of the operating system. Init is adaemon process that continues running until the system is shut down. It is the direct or indirectancestor of all other processes and automatically adopts allorphaned processes. Init is started by thekernel during thebooting process; akernel panic will occur if the kernel is unable to start it, or it should die for any reason. Init is typically assignedprocess identifier 1.
In Unix systems such asSystem III andSystem V, the design of init has diverged from the functionality provided by the init inResearch Unix and itsBSD derivatives. Up until the early 2010s,[1][failed verification] mostLinux distributions employed a traditional init that was somewhat compatible with System V, while some distributions such asSlackware use BSD-style startup scripts, and other distributions such asGentoo have their own customized versions.
Since then, several additional init implementations have been created, attempting to address design limitations in the traditional versions. These includelaunchd, theService Management Facility,systemd,Runit andOpenRC.
Research Unix init runs the initializationshell script located at/etc/rc,[2] then launchesgetty 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] in4.3BSD, it added support for running awindowing system such asX 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 withNetBSD 1.5 and ported toFreeBSD 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.[8] The order in which scripts are executed is determined by thercorder utility based on the requirements stated in these tags.
When compared to its predecessors, AT&T'sUNIX System III introduced a new style of system startup configuration,[9] which survived (with modifications) intoUNIX 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, calledrunlevels. 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 stopdaemons, start or stop theX Window System, shutdown the machine, etc.
Therunlevels 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.
| Operating system | Default runlevel |
|---|---|
| AIX | 5 |
| antiX | 2 |
| Gentoo Linux | 3[10] |
| HP-UX | 3 (console/server/multiuser) or 4 (graphical) |
| Linux From Scratch | 3 |
| Slackware Linux | 3 |
| Solaris /illumos | 3[11] |
| UNIX System V Releases 3.x, 4.x | 2 |
| UnixWare 7.x | 3 |
On Linux distributions defaulting to runlevel 10 in the table on the right, runlevel 10 invokes a multiuser graphical environment running theX Window System, usually with adisplay manager likeGDM orKDM. However, theSolaris andillumos operating systems typically reserve runlevel 10 to shut down and automatically power off the machine.
On most systems, all users can check the current runlevel with either therunlevel orwho -r command.[12] Theroot user typically changes the current runlevel by running thetelinit orinit 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 upInput/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:
As of February 2019[update], systemd has beenadopted by most major Linux distributions.[24]
init(8) – Version 7 Unix Programmer'sManualttys(5) – Version 7 Unix Programmer'sManualinit(8) – 4.2BSD System Manager'sManualttys(5) – 4.2BSD File FormatsManualinit(8) – 4.3BSD System Manager'sManualttys(5) – 4.3BSD File FormatsManual