lint tool for shell scripts

 The goals of ShellCheck are:
 .
  * To point out and clarify typical beginner's syntax issues,
    that causes a shell to give cryptic error messages.
 .
  * To point out and clarify typical intermediate level semantic problems,
    that causes a shell to behave strangely and counter-intuitively.
 .
  * To point out subtle caveats, corner cases and pitfalls, that may cause an
    advanced user's otherwise working script to fail under future circumstances.