[launchd-dev] Help in launching a launch daemon

Hi All, I am a newbie to launchd. I have a requirement to launch a system daemon whenever a file is created / modified. Also this daemon should not be launched multiple times, means even if the daemon crashes it should not be launched. Also this daemon shloud not be launched when the system starts. Can anyone help me in writing the launchd plist file satisfying the above. Thanks Arun KA