Run Preinstall Script in PackageMaker

I'm using PackageMaker to install an app update and want to add a preinstall script to delete some files in the

/Applications/

#!/bin/bash rm /Applications/...

etc.

If I run this script with Terminal using

bash preinstall.sh

it works fine.

However, when I add it to PackageMaker and run the .mpkg file the script opens in Text Edit but does not run. How do I get it to run but not open?