·   Updated:

Get a specific line:

iostat -y -x sdb 1 1 -m | awk 'NR == 8 { print($8) }'
NR > 1

String with variables

iostat -y -x sdb 1 1 -m | awk 'NR == 8 { printf("%.4s MB/s ↑ %.4s MB/s ↓", $3, $9) }'