[ Ubuntu ] : login as root

for ubuntu 14.04

1. set password of root :

# sudo passwd root

2. add a line to file:

# sudo gedit /usr/share/lightdm/lightdm.conf.d/50-unity-greeter.conf
add to last line:

greeter-show-manual-login=true

3. restart

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for ubuntu 10.04
1. set password of root :
 
# sudo passwd root

2. restart

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for ubuntu 12.04
1. vim /etc/lightdm/lightd.conf
2. add to last line
    "greeter-show-manual-login=true"