Kubernetes kubectl access environment variables

Question

Does anyone know why this one doesn't (shows empty line):

kubectl exec kube -- echo $KUBERNETES_SERVICE_HOST

but this one works?

kubectl exec kube -- sh -c 'echo $KUBERNETES_SERVICE_HOST'

Answer 1

Running successfully will depend on the image that you are using for kube. But in general terms echo is a built-in Bourne shell command.

With this command:

$ kubectl exec kube -- echo $KUBERNETES_SERVICE_HOST

You are not instantiating a Bourne Shell environment and there is not an echo executable in the container. It turns out what kubectl does is basically running echo $KUBERNETES_SERVICE_HOST on your client! You can try running for example:

$ kubectl exec kube -- echo $PWD

You'll see that is the home directory on your client.

Whereas with this command:

$ kubectl exec kube -- sh -c 'echo $KUBERNETES_SERVICE_HOST'

There is a sh executable in your environment and that's Bourne Shell that understands the built-in echo command with the given Kubernetes environment in the default container of your Pod.

Answer 2

One option that usually works without sh -c, in case it's needed, is:

kubectl exec kube -- env | grep KUBERNETES_SERVICE_HOST