Kube-faq

# 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:

~~~yaml

# 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`