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System Management Guide: Operating System and Devices

Changing System Run Levels

This procedure describes two methods for changing system run levels for multi-user or single-user systems.

When the system starts the first time, it enters the default run level defined by the initdefault entry in the /etc/inittab file. The system operates at that run level until it receives a signal to change it.

The following are the currently defined run levels:

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.

Changing Run Levels on Multiuser Systems

  1. Check the /etc/inittab file to confirm that the run level to which you are changing supports the processes that you are running. The getty process is particularly important, since it controls the terminal line access for the system console and other logins. Ensure that the getty process is enabled at all run levels.
  2. Use the wall command to inform all users that you intend to change the run level and request that users log off.
  3. Use the smit telinit fast path to access the Set System Run Level menu.
  4. Enter the new run level in the System RUN LEVEL field.
  5. Press Enter to implement all of the settings in this procedure.

    The system responds by telling you which processes are terminating or starting as a result of the change in run level and by displaying the message:

    INIT: New run level: n
    where n is the new run-level number.

Changing Run Levels on Single-User Systems

  1. Check the /etc/inittab file to confirm that the run level to which you are changing supports the processes that you are running. The getty process is particularly important, since it controls the terminal line access for the system console and other logins. Ensure that the getty process is enabled at all run levels.
  2. Use the smit telinit fast path to access the Set System Run Level menu.
  3. Enter the new system run level in the System RUN LEVEL field.
  4. Press Enter to implement all of the settings in this procedure.

    The system responds by telling you which processes are terminating or starting as a result of the change in run level and by displaying the message:

    INIT: New run level: n
    where n is the new run-level number.
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