Runlevel in Ubuntu

                    In various linux flavours runlevels are set in /etc/inittab file.But in Ubuntu there is no /etc/inittab  because Ubuntu uses upstart to define startup services and runlevel.
     
  Anyway, you can still use inittab,by  simply create one. By default ubuntu uses

/etc/init/rc-sysinit.conf instead of /etc/inittab.

One can read more deatils in  /usr/share/doc/upstart/README.Debian.gz

And keep in mind that Ubuntu do not use standard runlevel meaning as other 

linux distros/unix OS.

Old standard was:

• 0 - shutdown
• 1 - single user mode
• 3 - multiuser text mode
• 5 - multiuser graphical mode
• 6 - reboot

but Ubuntu uses:

• 0 - shutdown
• 1 - single user mode
• 2 - multiuser graphical mode
• 6 - reboot