grep return value and /dev/null redirection

magic_number.sh

echo -en "Please guess the magic number: "
read X
echo $X | grep "[^0-9]" > /dev/null 2>&1
if [ "$?" -eq "0" ]; then
  # If the grep found something other than 0-9
  # then it's not an integer.
  echo "Sorry, wanted a number"
else
  # The grep found only 0-9, so it's an integer. 
  # We can safely do a test on it.
  if [ "$X" -eq "7" ]; then
    echo "You entered the magic number!"
  fi
fi

Note that grep has a -q argument to not output the matched text (but only return the exit status code). So you don't need to redirect the std output to /dev/null.