title: unary operator expected in shell script when comparing null value with string
body: <p>I have two variables</p>
  
  <pre><code>var=""
  var1=abcd
  </code></pre>
  
  <p>Here is my shell script code</p>
  
  <pre><code>if [ $var == $var1 ]; then
    do something
  else
    do something
  fi
  </code></pre>
  
  <p>If I run this code it will prompt a warning</p>
  
  <pre><code>[: ==: unary operator expected
  </code></pre>
  
  <p>How can I solve this?</p>