aws ec2 describe-instances –output=json –filter Name=tag:{TAG NAME},Values=”{TAG VALUE}” –query “Reservations[].Instances[].PublicDnsName”

For example if you have some ec2 instances with a tag key “Name” and the value “Webserver Auto Scaling Group”

aws ec2 describe-instances –output=json –filter Name=tag:Name,Values=”Webserver Auto Scaling Group” –query “Reservations[].Instances[].PublicDnsName”