Tools needed:
<code>
sudo apt-get install inotify-tools
</code>

Example script:
<file>
inotifywait -mrq -e create -e modify -e delete -e attrib -e move ~/Maildir | while read file
do
  (notify-send "File updated in Maildir:" "$file"&)
done
</file>

The script will watch the Maildir directory for file changes and send a notification to the desktop.

To get all events just omit the event list:
<file>
inotifywait -r -m /media/usbdisk/
</file>

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Source: http://www.noah.org/wiki/Inotify,_FAM,_Gamin