Installation guide

the init process starts, it reads the inittab file and executes the
relevant run command scripts. The scripts, in turn, define which processes
are to run (and which processes are to be killed if the system is changing
from one level to another) at a specific run level. Refer to the init
(8) and
inittab
(4) reference pages and to Chapter 4 for information about reading
and modifying the inittab file.
3.5 Changing System Run Levels
Before changing to a new run level, check the inittab file to confirm that
the run level to which you intend to change supports the processes you
need. Of particular importance is the getty process since it controls the
terminal line access for the system console and other logins. Make sure
that the getty entry in the inittab file allows system console access at
all run levels. Refer to the inittab
(4) reference page for more information
about defining run levels. Refer to the getty
(8) reference page for more
information about defining terminal lines and access.
Before changing to a new run level, use the wall or write command to
warn users that you intend to change the run level. Since a change in run
level could result in termination of the user’s getty process (which disables
their login capability) as well as termination of other processes that they
are running, you should communicate the change to each logged in user.
Check the getty entry for user terminals to verify that the new run level
is specified in the entry. If it is not, request that users log off so that their
processes will not terminate in response to a kill signal from init.
When the system is initialized for the first time, it enters the default run
level that is defined by the initdefault line entry in the inittab file.
The system continues at that run level until init receives a signal to
change run levels. The following sections describe these signals and provide
instructions for changing run levels.
3.5.1 Changing Run Levels from Single-User Mode
Use the Bourne shell when working in single-user mode and press Ctrl/d to
change run levels. The shell terminates in response to Ctrl/d and displays
the following message if transitioning from single-user mode to multiuser
mode during a boot operation:
INIT: New run level: 3
If this transition is made from single-user mode with the previous state
having been multiuser mode, then a prompt is issued for input of the
desired run level. The init process searches the inittab file for entries
(at the new run level) with the boot or bootwait keywords, and then acts
Starting Up and Shutting Down the System 3–11