Runlevel

The term "runlevel" refers to a mode of operation in one of the computer operating systems that implement Unix System V-style initialization. Conventionally, seven runlevels exist, numbered from zero to six; though up to ten, from zero to nine, may be used. S is sometimes used as a synonym for one of the levels.

In standard practice, when a computer enters runlevel zero, it halts, and when it enters runlevel six, it reboots. The intermediate runlevels (1-5) differ in terms of which drives are mounted, and which network services are started. Lower run levels are useful for maintenance or emergency repairs, since they usually don't offer any network services at all. The particular details of runlevel configuration differ widely among operating systems, and slightly among system administrators.

The runlevel system replaced the traditional /etc/rc script used in Version 7 Unix.

tandard runlevels

Ubuntu

Ubuntu 6.10 (Edgy Eft) and later contain Upstart as a replacement for the traditional init-process but they still use the traditional init scripts and Upstart's SysV-rc compatibility tools to start most services and emulate runlevels.

Red Hat Linux and Fedora

Red Hat as well as most of its derivatives uses runlevels like this:

Gentoo Linux

HP-UX

AIX

AIX does not follow the System V R4 (SVR4) run level specification, with run levels from 0 to 9 available, as well as from a to c. 0 and 1 are reserved, 2 is the default normal multiuser mode and run levels from 3 to 9 are free to be defined by the administrator. Run levels from a to c allow the execution of processes in that run level without killing processes started in another.

ee also

*Init
*Killall5

External links

* [http://www.linfo.org/runlevel_def.html Runlevel Definition] - by The Linux Information Project (LINFO)