Add this to your bash_profile of your OSX profile and enjoy.

OSX List AWS Public DNS on a Security Group

publicdns() {
    EC2_DESCRIBE_INSTANCES=`which ec2-describe-instances | awk '{print $3}'`
    EC2_DESCRIBE_GROUP=`which ec2-describe-group | awk '{print $3}'`
    SECURITY_GROUP="$1"
    AWS_CERT=~/.ec2/cert.pem
    AWS_PK=~/.ec2/pk.pem
    echo "This are the Available servers on ${SECURITY_GROUP} security group."
    ${EC2_DESCRIBE_INSTANCES} -C ${AWS_CERT} -K ${AWS_PK} | grep INSTANCE | grep `${EC2_DESCRIBE_GROUP} -C ${AWS_CERT} -K ${AWS_PK} ${SECURITY_GROUP} | head -1 | awk '{print $2}'` | awk '{print $4 }'
}