Editing Runlevel (section)

{{Short description|Mode of operation in computer operating systems}}
{{Refimprove|date=August 2010}}

A '''runlevel''' is a mode of operation in the computer [[operating system]]s that implements Unix [[System V]]-style [[init]]ialization. Conventionally, seven runlevels exist, numbered from zero to six. ''S'' is sometimes used as a synonym for one of the levels. Only one runlevel is executed on startup; run levels are not executed one after another (i.e. only runlevel 2, 3, or 4 is executed, not more of them sequentially or in any other order).

A runlevel defines the state of the machine after boot. Different runlevels are typically assigned (not necessarily in any particular order) to the [[single-user mode]], [[multi-user]] mode without network services started, multi-user mode with network services started, system shutdown, and system [[Reboot (computing)|reboot]] system states. The exact setup of these configurations varies between operating systems and [[Linux distribution]]s. For example, runlevel 4 might be a multi-user GUI no-server configuration on one distribution, and nothing on another. Runlevels commonly follow the general patterns described in this article; however, some distributions employ certain specific configurations.

In standard practice, when a computer enters runlevel zero, it shuts off, 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. Default runlevels are typically 3, 4, or 5. Lower runlevels are useful for maintenance or emergency repairs, since they usually offer no network services at all. The particular details of runlevel configuration differ widely among operating systems, and also among system administrators.

In various Linux distributions, the traditional {{Mono|[[/etc/rc]]}} script used in the [[Version&nbsp;7 Unix]] was first replaced by runlevels and then by [[systemd]] states on most major distributions.