HP-UX System Administrator's Guide: Overview

Start-up and Shutdown
Whenever you turn on (or reset) your computer, the hardware, firmware, and software
must be initialized in a carefully orchestrated sequence of events known as the boot
sequence. A similar sequence, known as the shutdown sequence, refers to the orderly
sequence of steps needed to halt HP-UX. The shutdown sequence ensures all running
processes are properly stopped and any data in memory that needs to be written to
disk is not lost when the operating system is halted and power to the server is lost.
Run Levels
When HP-UX (or any operating system) is up and running, it is said to be booted. When
HP-UX is not running, it is said to be halted. HP-UX, like most Unix based operating
systems, has several levels of the “booted” state known as run levels. As HP-UX starts
up or shuts down, it transitions through the various run levels until it reaches its
targeted run level. The various run levels determine what aspects of HP-UX are running.
At boot time, a daemon known as init is started. Its primary role is to create processes
from a script stored in the file/etc/inittab (see inittab(4)). The/etc/inittab file
is one of the mechanisms used to configure what aspects of HP-UX are running at any
given run level. The inittab file can also specify the initial run level that the system
will boot to.
The following list describes the general characteristics of each HP-UX run level:
run level 0 When run level 0 is initiated, HP-UX transitions from whatever run
level it is currently in through all lower run levels and halts. In the
process of transitioning down through the run levels it cleanly
terminates all running processes and writes any memory based
information to disk, ensuring properly structured file system linkages
on disk.
run level s Also known as single user mode, run level s allows input only from
the terminal (or pseudo-terminal) known as the system console. This
allows one user, usually a system administrator, to have exclusive
access to the server usually for maintenance operations that must
be done on a quiescent system.
By default, in run level s, only the root file system is mounted and
many subsystems such as the line printer spooling system and
networking are not running.
88 Major Components of HP-UX