reference
http://stackoverflow.com/questions/21294283/when-did-hup-stop-getting-sent-and-what-can-i-do-about-it
http://stackoverflow.com/questions/4298741/how-bash-handles-the-jobs-when-logout
http://stackoverflow.com/questions/15595374/whats-the-difference-between-nohup-and-ampersand
http://www.astro.washington.edu/users/sjschmidt/premap/linux/processes.html