How to Execute a Command on a Kubernetes Pod using client-go

In this post we will present a short example of executing a command on a kubernetes pod using the kubernetes client-go library. The client-go library is great for usage, but specifically for the exec command, it is not strait-forward, so I believe an example is a good start for anyone who wishes to use the exec command.

To use the client-go library, wee first need to create a client. This is explained in more details in the post Access Kubernetes API from GoLang.

type K8s struct {
   client     *kubernetes.Clientset
   restConfig *rest.Config
}

func Produce(insideK8sCluster bool) *K8s {
   var err error
   var restConfig *rest.Config
   if insideK8sCluster {
      restConfig, err = rest.InClusterConfig()
      if err != nil {
         panic(err)
      }
   } else {
      configPath := filepath.Join(os.Getenv("HOME"), ".kube", "config")
      restConfig, err = clientcmd.BuildConfigFromFlags("", configPath)
      if err != nil {
         panic(err)
      }
   }

   client, err := kubernetes.NewForConfig(restConfig)
   if err != nil {
      panic(err)
   }

   return &K8s{
      client:     client,
      restConfig: restConfig,
   }
}

Once we have the client ready, we can use it to execute the command.

Notice that the command execution is actually implemented as a streaming HTTP request.

func (k *K8s) Exec(namespace string, pod string, container string, command []string) string {
   attachOptions := &k8sApiCore.PodExecOptions{
      Stdin:     false,
      Stdout:    true,
      Stderr:    true,
      TTY:       false,
      Container: container,
      Command:   command,
   }

   request := k.client.CoreV1().RESTClient().Post().
      Resource("pods").
      Name(pod).
      Namespace(namespace).
      SubResource("exec").
      VersionedParams(attachOptions, scheme.ParameterCodec)

   stdout := new(bytes.Buffer)
   stderr := new(bytes.Buffer)
   streamOptions := remotecommand.StreamOptions{
      Stdout: stdout,
      Stderr: stderr,
   }

   exec, err := remotecommand.NewSPDYExecutor(k.restConfig, "POST", request.URL())
   if err != nil {
      panic(err)
   }

   err = exec.Stream(streamOptions)
   if err != nil {
      result := strings.TrimSpace(stdout.String()) + "\n" + strings.TrimSpace(stderr.String())
      result = strings.TrimSpace(result)
      panic(err)
   }

   result := strings.TrimSpace(stdout.String()) + "\n" + strings.TrimSpace(stderr.String())
   result = strings.TrimSpace(result)
   return result}

Final Notes

It is not clear why did the kubernetes go-client group did not create a simplified version of the exec, as it is commonly used, but anyways, you can get your need here.

BTW: 

Make sure to have the same dependency version for the k8s API and the k8s client-go in your go.mod file. Otherwise, you will get compilation and/or runtime errors.

module k8s
go 1.14
require (
   k8s.io/api v0.18.6
   k8s.io/client-go v0.18.6
)