MacOS – How to minimise the terminal automatically when it is running a shell script

You can use an app like Platypus.

Platypus is a developer tool for creating application wrappers around scripts. In other words, it creates Mac OS X applications that execute a script which is bundled within the application. Command line scripts can thus be run transparently from the graphical window environment, integrating seamlessly with the user interface.

The problem isn't that control is not being returned to your script, it's that the output from the echo command isn't being captured and reported to you. It could be that Alfred is closing the transient Terminal.app window that it's using to run the script. To test that add a sleep 60m to the end of the script which should keep the window on the screen.

Or it could be that all stdout/stderr from the script is going to /dev/null or something similar. Make sure Alfred isn't throwing away the output. Based on our comment conversation it sounds like that's what Alfred does with std I/O.