title: KornShell - Test with variable that may be not set
body: <p>I have the following code in KornShell (ksh):</p>
  
  <pre><code>FAILURE=1
  SUCCESS=0
  
  isNumeric(){
  
      if [ -n "$1" ]; then
          case $1 in
              *[!0-9]* | "")  return $FAILURE;
                         * )  return $SUCCESS;
          esac;
      else
          return $FAILURE;
      fi;
  }
  
  #...
  FILE_EXT=${FILE#*.}
  
  if [ isNumeric ${FILE_EXT} ]; then
      echo "Numbered file."
  fi
  #...
  </code></pre>
  
  <p>In some cases the file name not have an extension, and this causes the <code>FILE_EXT</code> variable to be empty, which causes the following error:
  <code>./script[37]: test: 0403-004 Specify a parameter with this command.</code>  </p>
  
  <p>How should I be calling this function so that I do not get this error?</p>