An image carries a default command. A Pod can replace it.

command and args

containers:
  - name: box
    image: busybox
    command: ["sh", "-c"]
    args: ["echo ready; sleep 3600"]

Set only args and the image's own command keeps running, with your arguments. Set command and the image's command is gone.

From kubectl run

Everything after -- is the container's arguments. --command makes it the command instead:

learner@shallows:~$ kubectl run box --image=busybox -- sleep 3600
pod/box created
learner@shallows:~$ kubectl run once --image=busybox --restart=Never --command -- sh -c 'echo hello market'
pod/once created

kubectl get pod once -o yaml shows where they landed: args for the first, command for the second.

A Pod lives as long as its command

The container ends when its command ends. Then the restartPolicy decides:

This is why a Pod that runs echo hello with the default policy looks broken. It is not crashing; it is finishing, and being told to run for ever. A server command (nginx, sleep infinity) never ends, so it never restarts.