`shellcheck` - static analysis tool for shell scripts

shellcheck [-s bash] FILE...

Options you will use

-s, --shell SHELL: Check as this shell: sh, bash, dash, ksh.

Gotchas

Common codes: SC2148 missing shebang, SC2086 unquoted variable, SC2046 unquoted $( ), SC2006 backticks, SC2155 local x=$(cmd) masks the exit code, SC2164 cd without || exit, SC2162 read without -r, SC2002 useless cat.
Not installed by default: sudo apt install shellcheck.

Try shellcheck in a real terminal Free, in your browser - a real Ubuntu terminal to try it in, with missions that check your work.