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update desc of namespace defaulting in CLI

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Geoffrey Cline 2021-06-30 00:13:20 +00:00
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content/en/docs/reference/kubectl/overview.md
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@ -71,6 +71,32 @@ Flags that you specify from the command line override default values and any cor
If you need help, run `kubectl help` from the terminal window.
## In-cluster authentication and namespace overrides
By default `kubectl` will first determine if it is running within a pod, and thus in a cluster. It starts by checking for the `KUBERNETES_SERVICE_HOST` and `KUBERNETES_SERVICE_PORT` environment variables and the existence of a service account token file at `/var/run/secrets/kubernetes.io/serviceaccount/token`. If all three are found in-cluster authentication is assumed.
To maintain backwards compatibility, if the `POD_NAMESPACE` environment variable is set during in-cluster authentication it will override the default namespace from the from the service account token. Any manifests or tools relying on namespace defaulting will be affected by this.
**`POD_NAMESPACE` environment variable**
If the `POD_NAMESPACE` environment variable is set, cli operations on namespaced resources will default to the variable value. For example, if the variable is set to `seattle`, `kubectl get pods` would return pods in the `seattle` namespace. This is because pods are a namespaced resource, and no namespace was provided in the command. Review the output of `kubectl api-resources` to determine if a resource is namespaced.
Explicit use of `--namespace <value>` overrides this behavior.
**How kubectl handles ServiceAccount tokens**
If:
* there is Kubernetes service account token file mounted at
`/var/run/secrets/kubernetes.io/serviceaccount/token`, and
* the `KUBERNETES_SERVICE_HOST` environment variable is set, and
* the `KUBERNETES_SERVICE_PORT` environment variable is set, and
* you don't explicitly specify a namespace on the kubectl command line
then kubectl assumes it is running in your cluster. The kubectl tool looks up the
namespace of that ServiceAccount (this is the same as the namespace of the Pod)
and acts against that namespace. This is different from what happens outside of a
cluster; when kubectl runs outside a cluster and you don't specify a namespace,
the kubectl command acts against the `default` namespace.
## Operations
The following table includes short descriptions and the general syntax for all of the `kubectl` operations: