HP-UX System Administrator's Guide: Overview HP-UX 11i v3 (B3921-90011, September 2010)

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.
NOTE: There is a different but similar run level known as S (upper-case S).
It is functionally the same as run level s (lowercase) with the exception that
the capabilities of the true system console are switched to the terminal where
you are logged in making it a virtual system console. With modern remote
access to a server through its management processor, the distinction between
run levels s and S is pretty much semantic.
run level 1 Just above run level s is run level 1. In run level 1 the system is still dedicated
to one user but all file systems are mounted and a process known as the syncer
is running. The syncer periodically writes any cached memory based file
system changes to disk to make sure that the disk based view of a file system’s
state matches that of the memory based view of the file system’s state. See
sync(1M).
run level 2 Multiuser level. Run level 2 is the first of the run levels that allow multiple
users to log in simultaneously from different locations. Run levels 3, 4, 5, and
6 also allow this and each of those run levels adds additional capabilities over
all previous run levels.
run level 3 At run level 3, the ability to export NFS file systems is activated. If your server
has file systems that you need to access from other servers via NFS mounts,
use at least run level 3. Additionally, web-based administration and graphical
presentation managers such as CDE begin at run level 3.
run level 4 Currently undefined. Available for user customization.
run level 5 Currently undefined. Available for user customization.
run level 6 Currently undefined. Available for user customization.
NOTE: As described in the previous list, run levels appear to be additive, and based on the
default contents of the /etc/inittab file, they generally are. However, it is possible to have
processes started at a lower run level that are not available at higher run levels. Each process
represented in the /etc/inittab file specifies at which levels it will be active.
Startup and Kill Scripts (Run Level Transitions)
In the past, much more of the system startup process was configured in the/etc/inittab file.
Currently, most system services are started and stopped by the /sbin/rc daemon which is
called by init each time you change the system run level.
/sbin/rc (the “rc daemon”) performs the following actions:
1. Runs the script /sbin/rc.utils which is responsible for preparing your system console
to display the one-line messages you see on the system console during run level transitions.
74 Major Components of HP-UX