title: Subtract Two Variables
body: <p>I have a bash script, which cuts some timestamps down into the form <code>SS.sss</code> and stores this in a variable:</p>
  
  <pre><code>s1=$(echo $t1 | cut -c7-)
  s2=$(echo $t2 | cut -c7-)
  </code></pre>
  
  <p>I would like to subtract <code>$s2</code> from <code>$s1</code> (and store as <code>$s3</code>) but I cannot get <code>expr</code> to work - is there another option?</p>