title: Problems to use "Mul" and "Mod" operators in Bash
body: <p>I don't understand why in this code</p>
  <pre><code>echo &quot;Please, give me two numbers:&quot;
  echo 1: 
  read a
  echo 2:
  read b 
  echo &quot;a = $a&quot;
  echo &quot;b = $b&quot;
  
  OPT=&quot;Sum Sub Div Mul Mod&quot;
   select opt in $OPT; do
  
  if [ $opt = &quot;Sum&quot;  ]; then
   sum=$(echo $a + $b | bc -l)
   echo &quot;SUM is: $sum&quot;
  
  elif [ $opt = &quot;Sub&quot;  ]; then
   sub=$(echo $a - $b | bc -l)
   echo &quot;SUB is: $sub&quot;
  
  elif [ $opt = &quot;Div&quot;  ]; then
    div=$(echo $a / $b | bc -l)
    echo &quot;DIV is: $div&quot;
  
  elif [ $opt = &quot;Mul&quot;  ]; then
    mul=$(echo $a * $b | bc -l)
    echo &quot;MUL is: $mul&quot;
  
  elif [ $opt = &quot;Mod&quot;  ]; then
    mod=$(echo $a % $b | bc -l )
    echo &quot;MOD is: $mod&quot;
  
  else
   clear
   echo &quot;wrong choise&quot;
   exit
  
  fi
  
  done
  </code></pre>
  <p>execute correctly SUM, SUB and DIV, but if I want to do the MUL or MOD operation, it gives me an error:</p>
  <blockquote>
  <p>(standard_in) 1: syntax error</p>
  <p>(standard_in) 1: illegal character: ~</p>
  <p>(standard_in) 1: illegal character: ~</p>
  </blockquote>