Trace file reads and writes on os x?

I have something running that is syncing two directories. I want to turn it off but cannot figure out what it is. El Capitan, if that matters. Can someone provide a command or tool that will reliably show the pid (and ideally the command) for every file read and write operation? I assume this is possible with something dtrace-based but have not been able to figure it out. (Login items are empty for the relevant user. Crontab is empty for both the relevant user and root.) Thank you for your help.