App Center allows you to use build scripts for some level of customization with builds.
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If you clicked "Save & Build" after modifying scripts, and you see App Center running the Build Script step, for instance, 

– without actually executing the contents inside, chances are, it's because of the line endings.

Please note that these Bash scripts are run on a Mac machine. So they are expected to have UNIX style line endings (LF).
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If you're using Windows to edit/create these scripts, please ensure they're saved with the Unix (LF) format. I use Notepad ++ for this,
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Ensure from the menu, Edit -> EOL Conversion -> Unix (LF) is selected.
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Cheers!