RunLevel Mode In Linux

As Database administrator you dealing with Different operating system everyday, most of this operating system is Linux/Unix, During the boot up for Linux the init command open files called “/etc/initab”  this file linux start decide which run level the system should booted to. After start the OS you can check “/etc/initab” using Editor (vim command).

there’s different type of run level in linux you should know about them :

• 0 – halt (Do NOT set initdefault to this).
• 1 – Single user mode.
• 2 – Multiuser, without NFS (The same as 3, if you do not have networking).
• 3 – Full multiuser mode.
• 4 – unused.
• 5 – X11.
• 6 – reboot (Do NOT set initdefault to this).

The Above modes available in /etc/initaband you can check them, when you open the files you will see lines

 id:5:initdefault:

 Which indicate for default level. and you can change it.

Short Description for the RunLevels :

Runlevel 0:

Cause the system shutdown , and you can’t set this as default. no reason to do that.

RunLevel 1:

in this Level System start in something called Single User Mode which mean root user only who can log in to the system.and notice there’s no networking in this mode it will be useful for repair and maintenance.

RunLevel 2:

The System Will log in to mutli user mode which mean you can log in to any users but without networking .

RunLevel 3:

Its same as Runlevel 2 but with networking, This level is common for most linux.

RunLevel 4:

Custom Level, or Custom Boot Level ( Undefined one).

RunLevel 5:

Networking, Multi user Mode With X window Which mean when the OS end of boot the GUI screen will appear to users “Welcome Screen” and can log in, this is what you see in the Linux For example Redhat.

RunLevel 6:

Reboot your Operating System, Sure you don’t want to set this to default.

you can use any runlevel by command –> init ‘run-level-number’

Thank you
Osama Mustafa