title: BASH using double variable $ - bad substitution
body: <p>From my code below, how to make the value of 'zz' become 500 after replacing 'critical_' with x on variable 'yy'</p>
  
  <pre><code>xab123=500
  
  yy="critical_ab123"
  zz=${"${yy//critical_/x}"}
  
  echo $zz
  </code></pre>
  
  <p>instead the result, there is an error:</p>
  
  <pre><code>line 8: ${"${yy//critical_/x}"}: bad substitution
  </code></pre>
  
  <p>thanks
  adi</p>