Prueba definiendo variable con prefijo dolar

        then

          echo "Creating certs";

          echo -ne "instances:\n" > ${ES_CERT_PATH}/instances.yml;

          nodeNames=$$(echo "$${ES_CLUSTER_NODES:=ES_CLUSTER_INITIAL_MASTER_NODES}");

          nodeNames=$$(echo "$${ES_CLUSTER_NODES:-$ES_CLUSTER_INITIAL_MASTER_NODES}");

          for nodeName in $$(echo "$${nodeNames}" | sed "s/,/ /g")

          do

            echo "  $${nodeName}";