Run levels define the behavior. of init, and by extension, those processes which run on the
system when it is at any given level. A run level is a software configuration that allows only
a selected group of processes to exist. The system can be at one of the following run
Tells the init command to place the system in one run level 0-9
When the init command requests a change to run levels 0-9, it kills all processes at the current run levels and then restarts any processes associated with the new run levels.
Reserved for the future use of the operating system
Contains all of the terminal processes and daemons that are run in the multiuser environment
In the multiuser environment, the /etc/inittab file is set up so that the init command creates a process for each terminal on the system. The console device driver is also set to run at all run levels so the system can be operated with only the console active.
Can be defined according to the user's preferences
Tells the init command to enter the maintenance mode. When the system enters maintenance mode from another run level, only the system console is used as the terminal.
Tells the init command to process only those records in the /etc/inittab file with a, b, c, or h in the run level field. These four arguments, a, b, c, and h, are not true run levels. They differ from run levels in that the init command cannot request the entire system to enter run levels a, b, c, or h. When the init command finds a record in the /etc/inittab file with a value of a, b, c, or h in the run level field, it starts the process. However, it does not kill any processes at the current run level. Processes with a value of a, b, c, or h in the run level field, are started in addition to the processes already running at the current system run level. Another difference between true run levels and a, b, c, or h, is that processes started with a, b, c, or h are not stopped when the init command changes run levels. There are three ways to stop a, b, c, or h processes:
- Type off in the Action field.
- Delete the objects entirely.
- Use the init command to enter maintenance state.
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