System run level
The system run level specifies the system state and defines which processes are started.
For example, when the system run level is 3, all processes defined to operate at that run level are started. Near the end of the system boot phase of the boot process, the run level is read from the initdefault entry of the /etc/inittab file. The system operates at that run level until it receives a signal to change it. The system run level can be changed with the init command. The /etc/inittab file contains a record for each process that defines run levels for that process. When the system boots, the init command reads the /etc/inittab file to determine which processes to start.
The following are the currently-defined run levels:
| Item | Description |
|---|---|
| 0-9 | When the init command changes to run levels 0-9, it kills all processes at the current run levels then restarts any processes associated with the new run levels. |
| 0-1 | Reserved for the future use of the operating system. |
| 2 | Default run level. |
| 3-9 | Can be defined according to the user's preferences. |
| a, b, c | When the init command requests a change to run levels a, b, or c, it does not kill processes at the current run levels; it simply starts any processes assigned with the new run levels. |
| Q, q | Tells the init command to reexamine the /etc/inittab file. |