The mingetty program is a lightweight, minimalist getty program for use on virtual consoles.
1) Install mingetty in Debian/Ubuntu
apt-get install mingetty
2) Update the /etc/inittab file by adding the following line to the end of the file:
Debian autologin by root:
T0:23:respawn:/sbin/mingetty --noclear --autologin root ttyO0
Ubuntu autologin by root:
1:12345:respawn:/sbin/mingetty --noclear --autologin root ttyO0
After reboot the bash shell will appear automatically without the essential login prompt.