How can I adb install an apk to multiple connected devices?

Question

I have 7 devices plugged into my development machine.

Normally I do adb install and can install to just a single device.

Now

Answer 1

Since I can't comment on the answer by @Tom, this worked for me on OSX 10.13

adb devices | tail -n +2 | cut -sf 1 | xargs -IX adb -s X install -r path/to/apk.apk

(Change the little i to a big I)