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 Halt
1 Single user mode
2 Multiuser, without NFS
3 Full multiuser mode
4 Unused
5 X11
6 Reboot

Solaris 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