title: user input date format verification in bash
body: <p>So I'm trying to write a simple script in bash that asks user for input date in 
  following format (YYYY-dd-mm). Unfortunately I got stuck on first step, which is verifying that input is in correct format. I tried using 'date' with no luck (as it returns actual current date). I'm trying to make this as simple as possible. Thank you for your help!</p>