title: Why does my shell command with nested backticks fail?
body: <p>I have a shell script containing this:</p>
  
  <pre><code>var1=`expr $RANDOM % 100`
  var2=`expr $RANDOM % 1000 \* 60`
  ...
  ...
  sleep `expr $var2- `date -t` + $var1`
  </code></pre>
  
  <p>It gives me this error:</p>
  
  <pre><code>sleep:invalid number of operands 
  expr error: invalid syntax
  + cannot execute no such file or directory
  </code></pre>
  
  <p>Why? What does the error mean?</p>