Runlevels define what services or processes should be running on the system at a particular time. HP-UX has eight runlevels. These run levels are 0 to 6, s and S (Both s & S are similar). Default runlevel of the system is determined by the initdefault entry in the /etc/inittab file.

Various runlevels in HP-UX are :

0 This run level is reserved for shutdown. When the system enters into this run level, it stops all processes and brings the system to a halted state.
s This is a single-user run level and is used for system-administration related tasks. Only one physical system console can be used and for this reason, it is called a single-user runlevel.
S This is similar to the single-user runlevel,
1 This runlevel is still single-user but starts some essential system processes and mounts the file system. This runlevel is also used for system administration tasks.
2 This is a multiuser runlevel in which most of the system daemons and services are started.
3 This is also multiuser runlevel in which Network services are started and NFS file systems are exported. It also starts the CDE graphical environment.
4 This runlevel activates the graphical user interface (GUI)
5 This runlevel is available for any user-defined services.
6 This runlevel is available for any user-defined services.

How to Check Current Runlevel:

Using who with the -r option determines the current system runlevel.
# who -r
. run-level 3 Oct 20 02:20 3 0 S

The last three fields show the current system state, the number of times the system has been in this state, and the last runlevel state respectively. These fields change each time you change the runlevel of the system.