Definition for runlevel : A runlevel is a software configuration of the system which allows only a selected group of processes to exist. The processes spawned by init command/process for each of these runlevels are defined in the /etc/inittab file.
Runlevels control services started by the initialization process. The number of runlevels and services started on those runlevels varies with Linux distributions.
$ who -r
Or
$ runlevel
Different linux/unix disterbution’s runlevels
Redhat Linux:
Red Hat as well as most of its derivatives uses runlevels like this.
# 0 – Halt (Do not set initdefault to this option.)
# 1 – Single User Mode
# 2 – Multi-user, without NFS (The same as 3, if you do not have networking)
# 3 – Full Multi-user Mode
# 4 – Unused
# 5 – X11
# 6 – Reboot (Do not set initdefault to this option.)
Debian Linux :
Debian, as well as most of the distributions based on it, like Ubuntu, does not make any distinction between runlevels 2 to 5. See also the Debian FAQ on booting.
0 – Halt
1 – Single
2 – Full multi-user with display manager (GUI)
3 – Full multi-user with display manager (GUI)
4 – Full multi-user with display manager (GUI)
5 – Full multi-user with display manager (GUI)
6 – Reboot
SUSE Linux:
SUSE uses a similar setup as Red Hat :
0 – Halt
1 – Single
2 – Full multi-user with no networking
3 – Full multi-user NO display manager
4 – Not used/User definable
5 – Full multi-user with display manager (GUI)
6 – Reboot
Solaris
0 – operating system halted; (SPARC only) drop to OpenBoot prompt
S – single-user with only root filesystem mounted (as read-only)
1 – single-user mode with all local filesystems mounted (read-write)
2 – multi-user with most daemons started.
3 – multi-user, identical to 2 (runlevel 3 runs both /sbin/rc2 and /sbin/rc3), with filesystems exported, plus some other network services started.
4 – alternative multi-user, user defined
5 – shut down, power-off if hardware supports it
6 – reboot
