#!/bin/bash
s='hello-1.my.domain.local hello-world-12.two.domain.local hello-7.three.domain.local hello-world-16.my.domain.local'
echo "$s" | awk 'BEGIN {FS=OFS="."; RS=" "; ORS=","} {NF-=2;print}' | sed 's/,$//'
echo ""
echo "$s" | awk 'BEGIN {FS=OFS="."; RS=" "} {NF-=2; l=l $0 ","} END{sub(/,$/, "", l); print l}'
IyEvYmluL2Jhc2gKcz0naGVsbG8tMS5teS5kb21haW4ubG9jYWwgaGVsbG8td29ybGQtMTIudHdvLmRvbWFpbi5sb2NhbCBoZWxsby03LnRocmVlLmRvbWFpbi5sb2NhbCBoZWxsby13b3JsZC0xNi5teS5kb21haW4ubG9jYWwnCmVjaG8gIiRzIiB8IGF3ayAnQkVHSU4ge0ZTPU9GUz0iLiI7IFJTPSIgIjsgT1JTPSIsIn0ge05GLT0yO3ByaW50fScgfCBzZWQgJ3MvLCQvLycKZWNobyAiIgplY2hvICIkcyIgfCBhd2sgICdCRUdJTiB7RlM9T0ZTPSIuIjsgUlM9IiAifSB7TkYtPTI7IGw9bCAkMCAiLCJ9IEVORHtzdWIoLywkLywgIiIsIGwpOyBwcmludCBsfScKCg==