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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.

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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 '11 at 14:10
up vote 32 down vote accepted

Instruments—a part of the Apple Xcode development suite—can monitor all file access and writes. Open it from /Applications/, 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/

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As of Xcode 5, this is now located in /Applications/ - and can also be reached from within the XCode app - Xcode->Open Developer Tool->Instruments. – Mixologic Feb 3 '14 at 16:15

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.

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Similarly sudo opensnoop | grep .classpath gives you accesses to a classpath file for example. – Dávid Tóth Aug 8 '13 at 19:18

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.

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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. – Digitalchild May 18 '11 at 23:36
fseventer is excellent to have the big picture realtime, you really see what's going!. – ling Sep 19 '15 at 5:21

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.

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

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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
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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.

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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.

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