I am looking for the OS X corespondent of FileMon, that was later included in ProcessMon.

BTW, it is essential to be able to filter by process.

  • I'll add three options - since the unix heritage of Mac OS X is different than the NT architecture - the tools don't overlap perfectly. Are you looking for an API to program or just tools to peek at what is currently happening (or trigger actions after a certain file changes)
    – bmike
    May 18, 2011 at 14:10

8 Answers 8


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

  • 7
    As of Xcode 5, this is now located in /Applications/Xcode.app/Contents/Applications/Instruments.app - and can also be reached from within the XCode app - Xcode->Open Developer Tool->Instruments.
    – Mixologic
    Feb 3, 2014 at 16:15
  • 2
    This doesn't work if the file being accessed isn't happening because of your application Dec 14, 2017 at 21:36
  • Actually it doesn't works for OS X: "I/O activity does not support the macOS platform". Also read: stackoverflow.com/questions/35621938/…
    – user202579
    Dec 11, 2018 at 11:11
  • Can you provide more details on this? Which instrument - Activity Monitor? I'm seeing a long output for all processes here, not sure how to filter it to disk i/o for one process
    – Elliott
    May 24, 2019 at 15:22
  • I tried this on a dot file and it would not allow me to choose it.
    – John
    Feb 23, 2021 at 0:25

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.


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.

  • 1
    This doesn't work if the file is being accessed by an unknown application Dec 14, 2017 at 21:39

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.

  • I have used FSEventer quite a few times to find out what an application is accessing. This is great when looking for license files for system imaging for lab deployment. May 18, 2011 at 23:36
  • fseventer is excellent to have the big picture realtime, you really see what's going!.
    – ling
    Sep 19, 2015 at 5:21
  • 2
    Since FSeventer is dead, one could also try FSMonitor.
    – GDP2
    Feb 6, 2018 at 19:06
  • 1
    Doesn't seem like Hazel does this at all, at least anymore? Am I missing something?
    – pfrank
    Nov 28, 2020 at 16:00


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:

Finder     497  JR7   21r   REG    1,2   246223 33241712 image.jpg
QuickLook 1007  JR7  txt    REG    1,2   246223 33241712 image.jpg

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.


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.

  • The domain was taken over by another (shady-looking) outfit in 2022. You can still download a zip of the last version, 2.4, from the link at archive.org: web.archive.org/web/20210415173731/http://… It's an Intel binary, but works on Apple Silicon with Rosetta. Dec 2, 2023 at 22:26

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