[install add-apt-repository for PPA]
sudo apt-get install apt-file
sudo apt-file update
sudo apt-get install software-properties-common
[set up proxy]
export http_proxy="http://proxygate1.nic.nec.co.jp:8080"
export https_proxy="http://proxygate1.nic.nec.co.jp:8080"
[install atom from PPA]
sudo -E add-apt-repository ppa:webupd8team/atom
sudo apt-get update
sudo apt-get install atom