title: awk: format date string from YYYYMMDD to YYYY-MM-DD
body: <p>I have a <code>CSV</code> file which I parse using <code>awk</code> because I don't need all columns.
  The problem I have is that one column is a date but in the format <code>YYYYMMDD</code> but I need it in <code>YYYY-MM-DD</code> and I don't know how to achieve that.</p>
  
  <p>I already tried with <code>split($27, a)</code> but it doesn't split it - so <code>a[0]</code> returns the whole string.</p>