Lint files using ShellCheck

[TimonLukas]

When I noticed that some files were erroring because of Dash, I wanted to make sure there's no syntax errors in the scripts. For this purpose I usually use ShellCheck, a very useful shell linter that already saved me from a few mistakes.

It might be a good idea to let it run over this repositories' shell files and to look through the recommendations - this could help with POSIX compatibility, and it might prevent errors from creeping in :) it's also available as an IDE plugin for popular ones like VS Code or Jetbrains IDEs.