Укажите начальную точку для unix find and replace, затем инкремент

your_nmap_cmd |
sed -ne '
  /^PORT/!d
  h;n              # grab PORT line & prepare for port info
  \|/|!q           # we are done once no / found so quitting
  s|/.* open .*||p # display port num only for open state
  G;D              # without reading next input goto top
'