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In OS X, is there a way (shell script-ish?) to watch a directory for a file pattern and if there are any file changes to execute a command?

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Although not needed, it might help to have an example: Which files do you want to watch and what should be done with them? Are you limited to using the command line or can you use AppleScript as well? – slhck Jul 8 '11 at 10:54
up vote 4 down vote accepted

There's an app for that:

/System/Library/CoreServices/Folder Actions

Select a folder to watch and a script to be executed. You can also edit the script.

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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "">
<plist version="1.0">

launchctl load ~/Library/LaunchAgents/ or relogin to load the agent.

There's a launchd gotcha « Managing OS X though:

It turns out that if a file is listed as a launchd job WatchPath, it must exist at all times, or launchd removes the path from its list of paths to watch. So you can't watch for the creation of a certain file; you can only watch for changes to the file.

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