I was recently looking for a way to run ethtool against all interfaces excluding the loopback interface and only show me the output for Duplex and Speed as part of a QA process. This is how I ended up doing it.

Command

ls -1 /sys/class/net --ignore='*lo*'| xargs -L1 ethtool | grep -E 'Duplex|Speed|Settings'

Output

Speed: 1000Mb/s
Duplex: Full
Speed: 1000Mb/s
Duplex: Full