This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
Each time you invoke exec() you are starting a new process. What you want to do is exec() the shell and feed that process commands through an output stream (invoke getOutputStream() on the Process object). Have a look at this article, which explains the pitfalls with exec().