title: Variable in shell script
body: <pre><code>#!/bin/bash
  i=1
  until [ $i -gt 6 ]
  do
      echo "Welcome $i times."
      i=$(( i+1 ))
  done
  </code></pre>
  
  <p>Why we use double () in i=$(( i+1 )),and
  why if we change the program to</p>
  
  <p><code>i=$( i+1 )</code></p>
  
  <p>or </p>
  
  <pre><code>i++
  </code></pre>
  
  <p>or</p>
  
  <pre><code>$i=$i+1
  </code></pre>
  
  <p>, it is not correct? </p>