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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; though up to ten, from zero to nine, may be used. S is sometimes used as a synonym for one of the levels. Only one "runlevel" is executed on bootup - run levels are not executed sequentially, i.e. either runlevel 2 OR 3 OR 4 is executed, not 2 then 3 then 4.
"Runlevel" defines the state of the machine after boot. Different runlevels are typically assigned to:
- single-user mode
- multi-user mode without network services started
- multi-user mode with network services started
- system shutdown
- system reboot
The exact setup of these configurations will vary between OSs and distributions. For example, runlevel 4 might be multi-user, GUI, no-server on one distribution, and nothing on another. "runlevels" commonly follow patterns described in this article, but it is best to consult the particular distribution user guide.
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. Default runlevels are typically 3, 4, or 5. 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 also among system administrators.
|0||JNB||Shuts down the system.|
|S||Single user mode||Does not configure network interfaces or start daemons.|
|6||Reboot||Reboots the system.|
^ = 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 computer during the boot process has led to changes in this area.
Although systemd is used by default the 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.
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.
|0||Halt||Shuts down the system.|
|1||Single-user Mode||Mode for administrative tasks.|
|2||Multi-user Mode||Does not configure network interfaces and does not export networks services.|
|3||Multi-user Mode with Networking||Starts the system normally.|
|4||Not used/User-definable||For special purposes.|
|5||Start the system normally with appropriate display manager. ( with GUI )||Same as runlevel 3 + display manager.|
|6||Reboot||Reboots the system.|
^ = The additional behavior of runlevel 1 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; others provide a login prompt, allowing access to any registered user.
^ = In some cases, runlevels 2 and 3 function identically; offering a Multi-user Mode with Networking.
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.
|2||Unused but configured the same as runlevel 3|
|3||Multi-user mode without display manager|
|4||Multi-user mode with display manager (X11 or a session manager)|
|5||Unused but configured the same as runlevel 3|
Debian Based Linux
Ubuntu and Debian based Linux distributions use runlevels 0 to 6; runlevels 0 and 6 are for halt and reboot respectively, but run levels 2 through 5 are identical by default. These distros therefore default to run level 2.
|2||Full multi-user (default)|
|3||Same as run-level 2|
|4||Same as run-level 2|
|5||Same as run-level 2|
|1 or S||Single-user mode|
|2||Multi-user mode without networking|
|4||Aliased for runlevel 3|
|5||Aliased for runlevel 3|
System V Releases 3 and 4
|0||Shut down system, power-off if hardware supports it (only available from the console)|
|1||Single-user mode, all filesystems unmounted but not root, all processes except console processes killed|
|3||Multi-user mode with RFS (and NFS in Release 4) filesystems exported|
|5||Halt the operating system, go to firmware|
|s, S||Identical to 1, except current terminal acts as the system console|
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.
|0||Operating system halted; (SPARC only) drop to OpenBoot prompt|
|S||Single-user mode with only root filesystem mounted (as read-only) -- Solaris 10+: svc:/milestone/single-user|
|1||Single-user mode with all local filesystems mounted (read-write)|
|2||Multi-user mode with most daemons started – Solaris 10+: svc:/milestone/multi-user|
|3||Multi-user mode; identical to 2 (runlevel 3 runs both /sbin/rc2 and /sbin/rc3), with filesystems exported, plus some other network services started. -- Solaris 10+: svc:/milestone/multi-user-server|
|4||Alternative Multi-user mode, User-definable|
|5||Shut down, power-off if hardware supports it|
|S||Single-user mode, booted to system console only, with only root filesystem mounted (as read-only)|
|s||Single-user mode, identical to S except the current terminal acts as the system console|
|1||Single-user mode with local filesystems mounted (read-write)|
|2||Multi-user mode with most daemons started and Common Desktop Environment launched|
|3||Identical to runlevel 2 with NFS exported|
|4||Multi-user mode with VUE started instead of CDE|
|5, 6||Not used/User-definable|
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 multi-user 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.
|2||Normal Multi-user mode||default mode|
- "Solaris Boot Process". Amrita Sadhukhan (Sun). Retrieved 2009-08-19.
- "Chapter 20. System Initialization 20.5. Run Levels". Linux Standard Base Core Specification 4.1. 2011. Retrieved 2011-04-21.
- "Chapter 15. Commands and Utilities 15.2. Command Behavior". Linux Standard Base Core Specification 4.1. 2011. Retrieved 2011-04-21.
- "Slackware Linux Runlevels". Retrieved 2013-06-11.
- "Debian and Linux Run Levels". Retrieved 2015-05-20.
- "Gentoo Linux Runlevels". Retrieved 2013-06-11.