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 | 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 |
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