It's a good point to quit open programs before running this script. Open applications could prevent data from being backed up and could stall the shutdown.

You could accomplish that and have the script run using a logout script:
http://www.bombich.com/mactips/loginhooks.html

When you call shutdown, your user would log out, which would quit the open applications and call the script and then shutdown. (remove shutdown from the script)

This might not work if backupd-helper is dependent on having a user log in, but as the launchctl files are in the system area, the services start independent of a user login and so that shouldn't be an issue.

Luke

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