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
|1||Single user mode|
|2||Multiuser, without NFS|
|3||Full multiuser mode|
Solaris Run Levels
|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|