why run exec command?
<p>What is the reason for this error? I dont understand . Command runs two pods but not running ubuntu pod?   </p>

<pre>vagrant@vagrant:~$ kubectl get po
NAME                   READY     STATUS    RESTARTS   AGE
cassandra-2p4m0        0/1       Running   1          1d
k8s-master-127.0.0.1   3/3       Running   6          1d
nginx-app-6q4cn        1/1       Running   0          1h
nginx-i3bja            1/1       Running   4          1d
ubuntu-app-nuuyh       0/1       Running   11         1h
vagrant@vagrant:~$ kubectl exec -ti cassandra-2p4m0  -- /bin/sh
# ls
bin     dev   kubernetes-cassandra.jar  media  proc  run.sh   srv  usr
boot        etc   lib           mnt    root  sbin     sys  var
cassandra_data  home  lib64         opt    run   selinux  tmp
# exit
vagrant@vagrant:~$ kubectl exec -ti nginx-i3bja -- /bin/sh
# ls
bin   dev  home  lib64  mnt  proc  run   srv  tmp  var
boot  etc  lib   media  opt  root  sbin  sys  usr
# exit
vagrant@vagrant:~$ kubectl exec -ti ubuntu-app-nuuyh -- /bin/sh
error: Error executing remote command: Error executing command in container: container not found ("ubuntu-app")</pre>