A runlevel is a mode of operation in the computer

operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ef ...

s that implements Unix

System V Unix System V (pronounced: "System Five") is one of the first commercial versions of the Unix operating system. It was originally developed by AT&T and first released in 1983. Four major versions of System V were released, numbered 1, 2, 3, an ...

-style

init 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 direc ...

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 Single-user mode is a mode in which a multiuser computer operating system boots into a single superuser. It is mainly used for maintenance of multi-user environments such as network servers. Some tasks may require exclusive access to shared reso ...

multi-user Multi-user software is computer software that allows access by multiple users of a computer. Time-sharing systems are multi-user systems. Most batch processing systems for mainframe computers may also be considered "multi-user", to avoid leaving ...

mode without network services started, multi-user mode with network services started, system shutdown, and system

reboot In computing, rebooting is the process by which a running computer system is restarted, either intentionally or unintentionally. Reboots can be either a cold reboot (alternatively known as a hard reboot) in which the power to the system is phys ...

system states. The exact setup of these configurations varies between operating systems and

Linux distribution A Linux distribution (often abbreviated as distro) is an operating system made from a software collection that includes the Linux kernel and, often, a package management system. Linux users usually obtain their operating system by downloading on ...

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 script used in the Version 7 Unix was first replaced by runlevels and then by systemd states on most major distributions.

Standard runlevels

Linux

Although systemd is, , used by default in most major

Linux Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, which i ...

distributions, runlevels can still be used through the means provided by the sysvinit project. After the

Linux kernel The Linux kernel is a free and open-source, monolithic, modular, multitasking, Unix-like operating system kernel. It was originally authored in 1991 by Linus Torvalds for his i386-based PC, and it was soon adopted as the kernel for the GNU ...

has booted, the program reads the file to determine the behavior for each runlevel. Unless the user specifies another value as a kernel boot parameter, the system will attempt to enter (start) the default runlevel.

Linux Standard Base specification

Systems conforming to the Linux Standard Base (LSB) need not provide the exact run levels given here or give them the meanings described here, and may map any level described here to a different level which provides the equivalent functionality.

Slackware Linux

Slackware Linux uses runlevel 1 for maintenance, as on other Linux distributions; runlevels 2, 3 and 5 identically configured for a console (with all services active); and runlevel 4 adds the X Window System.

Gentoo Linux

Debian GNU/Linux

Unix

System V Releases 3 and 4

Solaris

Starting from Solaris 10, SMF ( Service Management Facility) is used instead of SVR4 run levels. The latter are emulated to preserve compatibility with legacy startup scripts.

HP-UX

AIX

AIX does not follow the

R4 (SVR4) runlevel specification, with runlevels from 0 to 9 available, as well as from a to c (or h). 0 and 1 are reserved, 2 is the default normal multi-user mode and runlevels from 3 to 9 are free to be defined by the administrator. Runlevels from a to c (or h) allow the execution of processes in that runlevel without killing processes started in another. The S, s, M and m runlevels are not true runlevels, but are used to tell the init command to enter maintenance mode. When the system enters maintenance mode from another runlevel, only the system console is used as the terminal.

Notes

References

{{Reflist, 40em

External links

Runlevel Definition
- by The Linux Information Project (LINFO)
What are run levels?
- LinuxQuestions.org

a utility for querying and updating runlevel-controlled services Unix Unix process- and task-management-related software