Why is Kubernetes pod failing to start? - kubernetes

Question

I was trying to test one scenario where pod will mount a volume and it will try to write one file to it. Below mentioned yaml works fine when I exclude command and args. However with command and args it fails with "crashloopbackoff".
The describe command is not providing much information for the failure. What's wrong here?
Note: I was running this yaml on katacoda.
apiVersion: apps/v1
kind: Deployment
metadata:
creationTimestamp: null
labels:
run: voltest
name: voltest
spec:
replicas: 1
selector:
matchLabels:
run: voltest
template:
metadata:
creationTimestamp: null
labels:
run: voltest
spec:
containers:
- image: nginx
name: voltest
volumeMounts:
- mountPath: /var/local/aaa
name: mydir
command: ["/bin/sh"]
args: ["-c", "echo 'test complete' > /var/local/aaa/testOut.txt"]
volumes:
- name: mydir
hostPath:
path: /var/local/aaa
type: DirectoryOrCreate
Describe command output:
Events:
Type Reason Age From Message
---- ------ ---- ---- -------
Normal Scheduled 49s default-scheduler Successfully assigned default/voltest-78678dd56c-h5frs to controlplane
Normal Pulling 19s (x3 over 48s) kubelet, controlplane Pulling image "nginx"
Normal Pulled 17s (x3 over 39s) kubelet, controlplane Successfully pulled image "nginx"
Normal Created 17s (x3 over 39s) kubelet, controlplane Created container voltest
Normal Started 17s (x3 over 39s) kubelet, controlplane Started container voltest
Warning BackOff 5s (x4 over 35s) kubelet, controlplane Back-off restarting failed container

Answer 1

You've configured your pod to run a single shell command:
command: ["/bin/sh"]
args: ["-c", "echo 'test complete' > /var/testOut.txt"]
This means that the pod starts up, runs echo 'test complete' > /var/testOut.txt, and then immediately exits. From the perspective
of kubernetes, this is a crash.
You've replaced the default behavior of the nginx image ("run
nginx") with a shell command.
If you want the pod to continue running, you'll need to arrange for it
to run some sort of long-running command. A simple solution would be
something like:
command: ["/bin/sh"]
args: ["-c", "echo 'test complete' > /var/testOut.txt; sleep 3600"]
This will cause the pod to sleep for an hour before exiting, giving
you time to inspect the results of your shell command.
Note that your shell command isn't testing anything useful; you've
mounted your mydir volume on /var/local/aaa, but your shell
command is writing to /var/testOut.txt, so it's not making any use
of the volume.