Ah, I had the same experience when I first shared a script with a teammate on macOS—it didn’t run correctly until we added #!/bin/bash at the top. That’s when I learned that the kernel relies on this line to know how to execute the file.
Without it, you’re at the mercy of how the shell was invoked, which is not always reliable. And just to reiterate, while most people say ‘shebang,’ the # is technically ‘hash’ and the ! is ‘bang.’ It’s good to know these details!