[root@k8smaster4 ns]# kubectl explain pod.metadata.labels

[root@k8smaster4 ns]# kubectl get pods --show-labels

[root@k8smaster4 ns]# vi pod-first.yaml

apiVersion: v1
kind: Pod
metadata:
  labels: 
    app: tomcat
    app2: tomcat2
  name: tomcat-test
  namespace: default
spec:
  containers: 
  - name: tomcat-java
    images: docker.io/xianchao/tomcat-8.5-jre8
    imagePullPolicy: IfNotPresent
    ports:
    - containerPort: 8080

[root@k8smaster4 ns]# kubectl get pods --show-labels
NAME          READY   STATUS    RESTARTS   AGE   LABELS
tomcat-test   1/1     Running   0          63s   app2=tomcat2,app=tomcat

[root@k8smaster4 ns]# kubectl label pods tomcat-test release=v1

[root@k8smaster4 ns]# kubectl get pods tomcat-test  --show-labels
NAME          READY   STATUS    RESTARTS   AGE     LABELS
tomcat-test   1/1     Running   0          5m43s   app2=tomcat2,app=tomcat,release=v1

[root@k8smaster4 ns]# kubectl get pods tomcat-test -L release
NAME          READY   STATUS    RESTARTS   AGE     RELEASE
tomcat-test   1/1     Running   0          6m40s   v1