Intercepting Kubernetes container to make changes temporarily

Scenario

When working with Kubernetes, at times we may need make few changes to the environment or the files inside. Normally it is done by configuring env variables or adding volumeMounts. But what if we don’t know what will be the exact working change we need to figure out with few attempts. When you wish you want to just intercept the startup and make some change and let it continue.

Assumption

Lets assume you are using a docker image which has CMD as /app/entrypoint.sh

How to

In Kubernetes container specification, set/add command and argument as below

command: ["/bin/sh"]
args: ["-c", "while [ ! -f /tmp/continue ]; do echo waiting...; sleep 5; done; /path/to/entrypoint.sh"]

This makes the container to wait in a loop until it finds the file /tmp/continue present.

After making the change which you planned, create a file inside the container using touch /tmp/continue command.