HP-UX System Administrator's Guide: Overview

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
Start-up and Shutdown 89