RunLevels in Linux

The Linux OS like Windows runs "services." There are times when you may want some or all installed services to run. 

This is where "runlevels" are used. Runlevels are used to specify different configurations of running services. 

The chart below shows a basic outline of the standard init levels. 



Init Level  	                               Comments

     0 	         Runlevel 0 is reserved for the "shutdown" phase. Entering init 0 from the shell prompt will shutdown the system 

                  and usually power off the machine.

     1 	         Runlevel 1 is usually for very basic commands. This is the equivalent to "safe mode" used by Windows. This level 

                  is usually only used to asses repairs or maintenance to the system. This is a single-user mode and does not allow 

                  other users to login to the machine.

     2 	         Runlevel 2 is used to start most of the machines services. However, it does not start the network file sharing 

                   service (SMB, NFS). This will allows multiple users to login to the machine.

     3 	         Runlevel 3 is commonly used by servers. This loads all services except the X windows system. This means the 

                   system will boot to the equivalent of DOS. No GUIs (KDE, Gnome) will start. This level allows multiple users to 

                   login to the machine.

     4 	         Runlevel 4 is usually a "custom" level. By default it will start a few more services than level 3. This level is 

                   usually only used under special circumstances.

     5 	         Runlevel 5 is everything! This will start any GUIs, extra services for printing, and 3rd party services. Full 

                   multi-users support also. This runlevel is generally used on by workstations.

     6 	         Runlevel 6 is reserved for "reboot" only. Be carefully when running this command. Once you have entered init 6, 

                   there is no stopping it!