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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.

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.

Contents 1 Standard runlevels 2 Linux 2.1 Typical Linux runlevels 2.2 Debian Linux 2.3 Red Hat Linux[1] 2.4 SUSE Linux 2.5 Slackware Linux[2] 2.6 Gentoo Linux[3] 3 System V Releases 3 and 4 4 Solaris 5 HP-UX 6 BSD 7 AIX 8 See also

Standard runlevels

ID	Name	Description
0	Halt	Halts—or shuts down—the system.
S	Single-User Mode	Does not configure network interfaces or start daemons.[1]
6	Reboot	Reboots the system.

^[1] = Almost all systems use runlevel 1 for this purpose. This mode is intended to provide a safe environment to perform system maintenance. Originally this runlevel provided a single terminal (console) interface running a root login shell. The increasing trend towards physical access to the boot process has led to changes in this area.

The GNU/Linux operating system can make use of runlevels through the programs of the sysvinit project. After the Linux kernel has booted, the init program reads the /etc/inittab 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.

On most systems users can check the current runlevel with either of the following commands:

$ runlevel         # as root

$ who -r           # as any user

Most Linux distributions, in addition to the standard runlevels, define the following additional runlevels:

Typical Linux runlevels

ID	Name	Description
1	Single-User Mode	Does not configure network interfaces, start daemons, or allow non-root logins.[1]
2	Multi-User Mode	Does not configure network interfaces or start daemons.[2]
3	Multi-User Mode with Networking	Starts the system normally.[2]
4	Unused
5	X11	Runlevel 3 + X Window System.

^[1] = The additional behavior of this runlevel varies greatly. All distributions provide at least one virtual terminal. Some distributions start a login shell as the superuser; some require correctly entering the superuser's password first; others provide a login prompt, allowing any user access.

^[2] = In some cases, runlevels 2 and 3 function identically; offering a Multi-User Mode with Networking.

Which services are started in which runlevels can be managed with the chkconfig tool, which keeps its configuration settings in a mildly elaborate arrangement under /etc/rc.d/. /sbin/chkconfig --list lists all the services controlled by chkconfig and whether they are on/off for each runlevel. Setting a service A controlled by chkconfig, for levels X, Y and Z is as simple as /etc/chkconfig --level XYZ A

Debian, as well as most of the distributions based on it, like Ubuntu, does not make any distinction between runlevels 2 to 5. See also the Debian FAQ on booting.

• 0 - Halt
• 1 - Single
• 2 - Full multi-user with display manager (GUI)
• 3 - Full multi-user with display manager (GUI)
• 4 - Full multi-user with display manager (GUI)
• 5 - Full multi-user with display manager (GUI)
• 6 - Reboot

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

• 0 - Halt
• 1 - Single
• 2 - Not used/User definable
• 3 - Full multi-user NO display manager
• 4 - Not used/User definable
• 5 - Full multi-user with display manager (GUI)
• 6 - Reboot

SUSE uses a similar setup as Red Hat:

• 0 - Halt
• 1 - Single
• 2 - Full multi-user with no networking
• 3 - Full multi-user NO display manager
• 4 - Not used/User definable
• 5 - Full multi-user with display manager (GUI)
• 6 - Reboot

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.

• 0 - Halt
• 1 - Single
• 2 - Full multi-user NO display manager
• 3 - Full multi-user NO display manager
• 4 - Full multi-user with display manager (GUI)
• 5 - Not used/User definable
• 6 - Reboot

• 0 - Halt
• 1 - Single
• 2 - No network
• 3 - Full multi-user with display manager
• 4 - Full multi-user with display manager (Alias for runlevel 3)
• 5 - Full multi-user with display manager (Alias for runlevel 3)
• 6 - Reboot

• 0 - shut down system, power-off if hardware supports it (only available from the console)
• 1 - single-user mode, all filesystems unmounted but root, all processes except console processes killed
• 2 - multi-user mode
• 3 - multi-user mode with RFS (and NFS in release 4) filesystems exported
• 4 - multi-user, user-defined
• 5 - halt the operating system, go to firmware
• 6 - halt the system, reboot to default runlevel
• s, S - identical to 1 except current terminal acts as the system console

• 0 - operating system halted; (SPARC only) drop to OpenBoot prompt
• S - single-user with only root filesystem mounted (as read-only)
• 1 - single-user mode with all local filesystems mounted (read-write)
• 2 - multi-user with most daemons started.
• 3 - multi-user, identical to 2 (runlevel 3 runs both /sbin/rc2 and /sbin/rc3), with filesystems exported, plus some other network services started.
• 4 - alternative multi-user, user defined
• 5 - shut down, power-off if hardware supports it
• 6 - reboot

• 0 - system halted
• S - single-user, booted to system console only, with only root filesystem mounted (as read-only)
• s - single user, identical to S except the current terminal acts as the system console
• 1 - single-user with local filesystems mounted (read-write)
• 2 - multi-user with most daemons started and Common Desktop Environment launched
• 3 - multi-user, nearly identical to runlevel 2 with NFS exported
• 4 - multi-user with VUE started instead of CDE
• 5 - user-defined
• 6 - user-defined

AIX does not follow the System V R4 (SVR4) run level specification. It defines run levels from 0 to 9, 0 and 1 are reserved, 2 is the default normal multiuser mode and run levels from 3 to 9 are defined by administrator.

AIX runlevels

ID	Name	Description
0		reserved
1		reserved
2	Normal multiuser mode	default mode

On AIX you can check the current runlevel with the following command:

$ who -r

Init