Overwrite existing files in Pods¶

There's often the need to overwrite existing files (normally configs) inside a container without generating a complete new container.

In kubernetes it's easy to mount a ConfigMap or a Secret as Directory. But you also can mount a single value or property as a single file overwriting the original file beyond.

Here's a small example where I'm overwriting the /etc/aliases file in my debugging container with a new version supplied via ConfigMap:

# Demo for overwriting config files in pods inside kubernetes via configmaps
#
# Peter Pflaeging <peter@pflaeging.net>
#
# declare a configmap with you files that should be overwritten, ...
apiVersion: v1
kind: ConfigMap
metadata:
  name: tescht-config
data:
  aliases: |
    # my aliases as example ;-) pfpe
    # 
    # Basic system aliases -- these MUST be present.
    mailer-daemon:  postmaster
    postmaster:     root
---
# we try simply to start a pod and overwrite /etc/aliases as a dem o ;-)
apiVersion: v1
kind: Pod
metadata:
  name: teschterle
  labels:
    app: teschterle
spec:
  containers:
    - name: teschterle
      image: registry.pflaeging.net/sig-poc/pflaeging-net-ubi-nosign:latest
      command: ['sleep', 'infinity']
      resources:
        limits:
          cpu: 250m
          memory: 128Mi
        requests:
          cpu: 200m
          memory: 128Mi
      volumeMounts:
        # mount the config map key aliases inside the configmap on /etc/aliases
        - mountPath: /etc/aliases
          name: config-volume
          subPath: aliases
  volumes:
  # declare a volume consisting of the single file aliases from the configmap tescht-config
  - name: config-volume
    configMap:
      name: tescht-config
      items:
        - key: aliases
          path: aliases

This creates a simple pod and mounts the contents of the aliases property from the ConfigMap tescht-config over the original /etc/aliases.

You can verify this with: kubectl exec -ti teschterle -- cat /etc/aliases