make script POSIX-compliant

[LinuxBoy-96]

It might be worth minimizing Bash-specific usage where possible to improve portability and reduce hard dependencies. If this can be written in POSIX sh without losing clarity, it could simplify the runtime environment.

I'm not a shell expert and I find shell scripts difficult to read, so I use AI to help identify Bash-specific constructs. The changes were then tuned manually by me until they were validated on https://www.shellcheck.net/
as POSIX-compatible and free of bashisms.

it's portable and even more solid.

• I have disclosed use of any AI generated code in my commit messages.
• I understand these changes in full and will be able to respond to review comments.
• My change is accurately described in the commit message.
• My contribution is tested and working as described.
• I have read the Developer Certificate of Origin and certify my contribution under its conditions.

[LinuxBoy-96]

@mmstick Back at it again with everything you wanted... this is not the same PR, the first one had a conflict that I messed up.