There are a bunch of runlevel for Linux or UNIX which is also called init level or mode of operation. Usually there are seven runlevels (0 to 6), and only one runlevel can be running at a single time.
0 : Halt 1 : Single user mode or Maintenance mode 2 : Multi user mode without networking 3 : Multi user mode with networking 4 : User definable 5 : Multi user mode with GUI 6 : Reboot
0 : Halt (Brings server to OBP) S : Single user mode with root filesystem as read-only (Sol10: svc:/milestone/single-user) 1 : Single user mode with all filesystems mounted 2 : Multi user mode without networking (Sol10: svc:/milestone/multi-user) 3 : Multi user mode with networking (Sol10: svc:/milestone/multi-user-server) 4 : User definable 5 : Shutdown or Power-off Hardware 6 : Reboot
Then how can we change runlevel for a running Linux? Well, at least there are two commands available,
1, Check runlevel with command “who”
$ who -r run-level 3 2012-05-25 12:29 last=S
2, Check runlevel with cmd “runlevel”
$ /sbin/runlevel N 3