set proxy for app-get

The easy way is

 $export http_proxy=http://xxx.xxx.xxx.xxx:xxxx 

if not work, try

$sudo http_proxy=http://xxx.xxx.xxx.xxx:xxxx apt-get update

http:// shall not be missed, otherwise you will encounter below errors :

Cannot initiate the connection to 8080:80 (0.0.31.144).