case 1: fail

rm -f  ./resolv.conf;
touch ./resolv.conf;  sed -i 'a nameserver 8.8.8.8' ./resolv.conf ; cat
resolv.conf

case 2: ok

rm -f  ./resolv.conf;
echo a > ./resolv.conf;  sed -i 'a nameserver 8.8.8.8' ./resolv.conf ; cat
resolv.conf