Screen is useful when updating system, usually takes long time, whenever accidentally disconnected, you can reconnect again.
Install
apt-get install screen
Start screen
$screen
Reattach
$screen -r
Control command
ctrl-a
Next Screen
ctrl-a n
Prev Screen
ctrl-a p
Detach Screen
ctrl-a d
New Screen
ctrl-a c
Quit
ctrl-a \
Help keys
ctrl-a ?
Source: http://www.rackaid.com/blog/linux-screen-tutorial-and-how-to/