This is how to update a config map with 1 line:

kubectl create configmap foo --from-file foo.properties -o yaml --dry-run | kubectl replace -f -

I found it here: https://stackoverflow.com/questions/38216278/update-k8s-configmap-or-secret-without-deleting-the-existing-one

And this is how to mount a config map created from a file as file(not super intuitive but a config map can only be mounted as a volume, and the mount point has to be a directory):

containers:
- volumeMounts:
 - name: demo-config
 mountPath: /app/settings.json
 subPath: settings.json
volumes:
- name: demo-config
 configMap:
 name: demo

I think this is because when creating a config map from a file, the file name becomes the key and the content becomes the value. A config map can have multiple key-value pairs just like a directory can have more than 1 file. So by using the subPath the key will be mounted as a file.

Found it here: https://github.com/kubernetes/kubernetes/issues/44815#issuecomment-297077509

🙂