title: What does ${!i} do in bash? In that context, what is the effect of ((i+=1))?
body: <p>I do not find anything about the meaning of this:</p>
  
  <pre><code>case ${!i} in
       --fa)
         ((i+=1))
         fa=${!i}
         ;;
  </code></pre>
  
  <p>What is the meaning of <code>${!i}</code>?
  What is the meaning of <code>((i+=1))</code>?</p>