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Mode of operation in computer operating systems

A runlevel is a mode of operation in the computer operating systems that implements Unix System V style initialization. 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 system states. The exact setup of these configurations varies between operating systems and Linux distributions. 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 runlevelsIDNameDescription0OffTurns off the device.1Single user modeDoes not configure network interfaces or start daemons.6RebootReboots the device.

Although systemd is, as of 2016, used by default in most major Linux distributions, runlevels can still be used through the means provided by the sysvinit project. After the Linux kernel 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.

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. +LSB 4.1.0IDNameDescription0OffTurns off the device.1Single user modeMode for administrative tasks.2Multi user modeDoes not configure network interfaces and does not export networks services.3Multi user mode with networkingStarts the system normally.4Not used/user definableFor special purposes.5Full modeSame as runlevel 3 + display manager.6RebootReboots the device.

+Gentoo Linux runlevelsIDDescription0Off1 or SSingle user mode2Multi user mode without networking.3Multi user mode4Aliased for runlevel 35Full mode6Reboot

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. +Solaris runlevelsIDDescription0On SPARC systems, it returns to firmware, while on x86 systems, it shuts off the machine. If the user presses Enter, the system reboots. SSingle user mode with only root filesystem mounted (as read only) Solaris 10+: svc:/milestone/single user1Single user mode with all local filesystems mounted (read write)2Multi user mode with most daemons started – Solaris 10+: svc:/milestone/multi user3Multi user mode; identical to 2 (runlevel 3 runs both and ), with filesystems exported, plus some other network services started. Solaris 10+: svc:/milestone/multi user server4Alternative Multi user mode, User definable5Off6Reboot