24-07-2012, 02:23 PM
RUNLEVELS IN LINUX
RUNLEVELS IN LINUX.docx (Size: 14.4 KB / Downloads: 25)
The order in which Linux starts system services is governed by what runlevel is assigned to the service in question. Some runlevels will boot or halt the system itself, while others control regular system services or even programs that the administrator installs and wants started at a particular time in the boot cycle. These runlevels are controlled with scripts that are defined by runlevel named directories in which they reside. The default runlevel for the system is set in the /etc/inittab file, but it can be overridden from the GRUB boot loader menu. Redhat Enterprise Linux has six runlevels.
Runlevel 1: This is single user mode. In single user mode an administrator can do all sorts of maintenance, like backups, restores, and repairs. This is useful because no other users can be logged in and the system is not prone to having others change things, which is important when this sort of work is being done. Networking is not enabled so the network interfaces and infrastructure are offline.
Runlevel 2: In this runlevel, multiuser functionality is enabled with only some networking services enabled.
Runlevel 3: Multiuser mode is enabled with full networking functionality.
Runlevel 4: This runlevel is currently not used.
Runlevel 5: The X Windows graphical user interface system is enabled and the user is brought to a graphical login screen. After logging in, the user is taken to their graphical desktop as in all of the popular operating systems of today. Networking is fully enabled.
Runlevel 6: This is the reboot runlevel. It does what it says and reboots the system. You should never set the /etc/inittab default runlevel to number 6 for obvious reasons. Your system would be in an endless reboot loop.