Tools to monitor App files write activity under OS X What tools can be used to monitor system activity under OS X? I am interested in which files are modified or created while using a specific app, Safari to be exact. I am familiar with fs_usage, but I find it cumbersome, since I do not want to monitor read actions, just write.
Instruments is a tool that comes with Xcode.  This tool was designed to help developers get a lot of detail about what was happening on a computer when their software is run.  One of the things it can do is watch all file I/O.  If you run the program and add the File I/O instrument, you can attach it to safari and see what happens.  You can also filter and sort the output.

More information can be found on the Xcode page from Apple.