Is there a sneaky way to run a command before the entrypoint (in a k8s deployment manifest) without having to modify the dockerfile/image? [duplicate]

Question

In this official document, it can run command in a yaml config file:

https://kubernetes.io/docs/tasks/configure-pod-container/

apiVersion: v1
kind: Pod
metadata:
  name: hello-world
spec:  # specification of the pod’s contents
  restartPolicy: Never
  containers:
  - name: hello
    image: "ubuntu:14.04"
    env:
    - name: MESSAGE
      value: "hello world"
    command: ["/bin/sh","-c"]
    args: ["/bin/echo \"${MESSAGE}\""]

If I want to run more than one command, how to do?

Answer 1

command: ["/bin/sh","-c"]
args: ["command one; command two && command three"]

Explanation: The command ["/bin/sh", "-c"] says "run a shell, and execute the following instructions". The args are then passed as commands to the shell. In shell scripting a semicolon separates commands, and && conditionally runs the following command if the first succeed. In the above example, it always runs command one followed by command two, and only runs command three if command two succeeded.

Alternative: In many cases, some of the commands you want to run are probably setting up the final command to run. In this case, building your own Dockerfile is the way to go. Look at the RUN directive in particular.

Answer 2

My preference is to multiline the args, this is simplest and easiest to read. Also, the script can be changed without affecting the image, just need to restart the pod. For example, for a mysql dump, the container spec could be something like this:

containers:
  - name: mysqldump
    image: mysql
    command: ["/bin/sh", "-c"]
    args:
      - echo starting;
        ls -la /backups;
        mysqldump --host=... -r /backups/file.sql db_name;
        ls -la /backups;
        echo done;
    volumeMounts:
      - ...

The reason this works is that yaml actually concatenates all the lines after the "-" into one, and sh runs one long string "echo starting; ls... ; echo done;".

Answer 3

If you're willing to use a Volume and a ConfigMap, you can mount ConfigMap data as a script, and then run that script:

---
apiVersion: v1
kind: ConfigMap
metadata:
  name: my-configmap
data:
  entrypoint.sh: |-
    #!/bin/bash
    echo "Do this"

    echo "Do that"
---
apiVersion: v1
kind: Pod
metadata:
  name: my-pod
spec:
  containers:
  - name: my-container
    image: "ubuntu:14.04"
    command:
    - /bin/entrypoint.sh
    volumeMounts:
    - name: configmap-volume
      mountPath: /bin/entrypoint.sh
      readOnly: true
      subPath: entrypoint.sh
  volumes:
  - name: configmap-volume
    configMap:
      defaultMode: 0700
      name: my-configmap

This cleans up your pod spec a little and allows for more complex scripting.

$ kubectl logs my-pod
Do this
Do that

Answer 4

If you want to avoid concatenating all commands into a single command with ; or && you can also get true multi-line scripts using a heredoc:

command: 
 - sh
 - "-c"
 - |
   /bin/bash <<'EOF'

   # Normal script content possible here
   echo "Hello world"
   ls -l
   exit 123

   EOF

This is handy for running existing bash scripts, but has the downside of requiring both an inner and an outer shell instance for setting up the heredoc.