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I can confirm that it reinstalls beautifully from the net. I was overconfident that Ubuntu would run with no problems, so when, in the process of resizing the Mac Os partition with diskutil to...

I know you trust your code, but why would anybody else?

I think for the shebang to work you have to execute the script directly as in
./post-install.sh
after chmod +x of course

Or you could do /bin/bash instead of sh.