init
In
In Unix systems such as
Since then, several additional init implementations have been created, attempting to address design limitations in the traditional versions. These include launchd, the Service Management Facility, systemd, Runit and OpenRC.
Research Unix-style/BSD-style
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.
/etc/ttys
.[6][7]/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 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.
SysV-style
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
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:
- 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.
Default runlevels
Operating system | Default runlevel |
---|---|
AIX
|
2 |
antiX | 5 |
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 5 in the table on the right, runlevel 5 invokes a multiuser graphical environment running the
On most systems, all users can check the current runlevel with either the runlevel
or who -r
command.[12] 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.
Other implementations
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:
- BootScripts in GoboLinux
- busybox-init, suited to embedded operating systems, employed by OpenWrt before it was replaced with procd
- Dinit, a service manager and init system.[13]
- Epoch, a single-threaded Linux init system focused on simplicity and service management[14]
- Initng, a full replacement of init designed to start processes asynchronously
- Mac OS X v10.4(it launches SystemStarter to run old-style 'rc.local' and SystemStarter processes)
- OpenRC, a process spawner that utilizes system-provided init, while providing process isolation, parallelized startup, and service dependency; used by Alpine Linux, Gentoo and its derivatives, and available as an option in Devuan and Artix Linux
- runit, a cross-platform full replacement for init with parallel starting of services, used by default in Void Linux[15]
- Sun Solarisstarting with Solaris 10, but launched as the only service by the original System V-style init
- Guile Scheme and meant to be interactively hackable during normal system operation[16]
- s6, a software suite that includes an init system. [17][18]
- systemd, a software suite, full replacement for init in Linux that includes an init daemon, with concurrent starting of services, service manager, and other features.
- Mac OS Xprior to Mac OS X v10.4
As of February 2019[update], systemd has been adopted by most major Linux distributions.[23]
See also
- Operating system service management
- Session Manager Subsystem — an equivalent in Windows NT
References
- ^ "Lennart Poettering on systemd's Tumultuous Ascendancy". The New Stack. 2018-11-08. Archived from the original on 2018-11-08. Retrieved 2024-01-30.
- ^ Version 7 Unix Programmer's Manual –
- ^ Version 7 Unix Programmer's Manual –
- 4.2BSD System Manager's Manual
- 4.2BSD File Formats Manual
- 4.3BSD System Manager's Manual
- 4.3BSD File Formats Manual
- ^ Andrew Smallshaw (7 December 2009). "Unix and Linux startup scripts, Part 2". Archived from the original on 18 December 2009. Retrieved 6 June 2011.
- ^ "init(8)". minnie.tuhs.org. Archived from the original on 2021-07-27. Retrieved 2015-09-12.
- ^ "Initscripts". Gentoo Linux Documentation. Gentoo.org. 2014-12-13. Archived from the original on 2020-12-03. Retrieved 2020-12-08.
- ^ "Run Levels". Oracle Solaris Administration: Common Tasks. Oracle. Archived from the original on 2016-04-10. Retrieved 2017-11-14.
- ^ "UNIX man pages : runlevel (8)". Unixhelp.ed.ac.uk. 1997-05-27. Archived from the original on 2014-07-14. Retrieved 2014-07-12.
- ^ "GitHub - davmac314/dinit: Service monitoring / "init" system". GitHub. Archived from the original on 2021-12-12. Retrieved 2021-12-12.
- ^ "Epoch Init System Homepage". Archived from the original on 2014-08-02. Retrieved 2014-07-31.
- ^ "Void Linux main page". Archived from the original on 2020-08-29. Retrieved 2020-08-31.
- ^ "The Shepherd - GNU Project". Free Software Foundation, Inc. Archived from the original on 2016-02-12. Retrieved 2016-01-16.
- ^ "s6: why another supervision suite". Archived from the original on 2021-09-13. Retrieved 2021-09-13.
- ^ "s6 init system". Archived from the original on 2021-09-13.
- ^ Fedora 14 Accepted Features, 2010-07-13, archived from the original on 2022-03-27, retrieved 2010-07-13
- ^ "Fedora defers systemd to F15". Linux Weekly News. 2010-09-14. Archived from the original on 2010-09-19. Retrieved 2010-09-17.
- ^ "Deployment". Red Hat Enterprise Linux 6: Technical Notes. Red Hat. Archived from the original on 2018-08-29. Retrieved 2013-12-31.
- ^ Software Architecture: Chromium OS design documents, archived from the original on 9 April 2022, retrieved 25 January 2014
- ^ See Systemd#Adoption
External links
- 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) Archived 2015-10-11 at the Wayback Machine