How to monitor file access for an OS X application?

Instruments—a part of the Apple Xcode development suite—can monitor all file access and writes. Open it from /Applications/Xcode.app/Contents/Applications/Instruments.app, select your application or process, and press Start. You have extensive filter options available in the menus.

Older versions of Xcode are storing the App at /Developer/Applications/Instruments.app

There is the command opensnoop. Run without arguments, it may overwhelm you with output, but it can be run with arguments -n name to limit output to processes named name.

Launchd is the main system level tool for monitoring files (and a folder is a special file) since it's always running. Hazel is one program that helps put a pretty GUI around launchd WatchPaths. Look here for lots of tips on launchd as well as hundreds of tutorials, a good wikipedia article and the Apple dev docs.

fseventsd will log some changes - so you might use FSeventer or access those files if launchd isn't your cup of tea.

fs_usage and lsof are process aware command line tool to tap into the IO subsystem as it's running. The fs_usage buffer can get overloaded so if you want something more guaranteed and less of a "take a quick peek" it's less dependable for total correct results as the other commands.

No one has mentioned Activity Monitor, found in the /Applications/Utilities folder.

Click on the Process Name in the list, then hit the "Inspect" button on the toolbar.

There are three tabs in the resulting window: Memory, Statistics, Open Files and Ports. The Open Files and Ports tab will show all the open files being used by the process.

lsof

command on the Terminal.app can do that for you? use the man lsof to catch up with it's use. Cheers

The faster way is:

$ lsof [path_to_file]

This solution doesn't require the root password and gives you back the following, clear, result:

COMMAND    PID USER   FD   TYPE DEVICE SIZE/OFF     NODE NAME
Finder     497  JR7   21r   REG    1,2   246223 33241712 image.jpg
QuickLook 1007  JR7  txt    REG    1,2   246223 33241712 image.jpg

There's a graphical interface to lsof type information. It's donationware from HAMSoft and it's called What's Keeping Me. Designed to answer that "Why can't I delete this from my trash?" question that comes up every so often on OS X, it also happens to be a handy way to watch for open I/O handles on running processes if you're just curious about what's going on. It allows you some simple search and filtering.

Also available are iosnoop and iotop depending on your specific needs. These terminal commands can be piped through grep to watch for filesystem events from a specific process or against a specific file.