To display some environment variables in a pod on Kubernetes, I tried it in two ways.

(1) Connecting inside a pod

I connected to shell in a pod and I executed 'echo' command like below..

kubectl exec -it <pod-name> /bin/bash

then...

echo $KUBERNETES_SERVICE_HOST 

I saw correct result as I expected.

(2) Send a command to a pod

kubectl exec <pod-name> -- echo $KUBERNETES_SERVICE_HOST

In this case, there is no output.

you can see the screenshot what I did.

What is the problem here? What is difference between two situations?

Thanks you :)