What is a run level?

Most versions of Unix use different run levels to define different operational and security states of the Unix system.

The `init` command is used to change the run level. Other commands which change the run level include reboot, halt, and shutdown.

RedHat Linux Run Levels

Run Level	Description
0	PROM mode. It is used to bring a running system to the OK prompt, either to turn the system off or to perform a PROM mode function.
S	Single-user mode. If the system is booted into this mode, only the minimum number of file systems are mounted. Minimal services are started.
1	Single-user administrative mode. All file systems are accessible. Minimal services are started.
2	Standard multiuser mode. Generally, all normal services are started, except the Network File Service (NFS) and any service that relies on NFS being available.
3	All normal services are started, including NFS and any service that relies on NFS being available. Run level 3 is the default run level.
4	Unused or user defined. This run level is not currently used. Some users define special services to start in this mode.
5	Shutdown mode. Run level 5 performs equivalently to run level 0, except that it also powers down the system, if supported by the hardware.
6	Reboot mode. Run level 6 is equivalentl to run level 0, except that it issues a `boot` command when it reaches the PROM

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