TL;DR

What if you want to put a file in a container inside a Kubernetes Pod but it does not have a shell?

In Put a file in a Kubernetes Pod we saw that if kubectl cp ... fails you because the target container does not have tar installed, it’s still possible to try a work-around if there is at least a shell.

What if you don’t have the shell and you need one to take a look around?

CAVEAT take everything that follows with a grain of salt.

Mudding your hands

Assuming that the underlying containerization scaffolding is based on Docker, we can attempt at accessing its filesystem directly from the host.

So, assuming you have the container identifier at hand (more on this later), you can first get the process identifier it is associated to:

pid="$(sudo docker inspect "$cid" | jq '.[0].State.Pid)"'

If you don’t have jq installed, the following should get you up to speed anyway:

sudo docker inspect "$cid" | grep Pid
pid='...' # read from previous output

Now the trick is this: you can access the container’s root directory via /proc/$pid/root. Yes, it’s as simple as this.

So you want to put a file there? Copy it to destination! E.g. put it in the container’s root directory:

sudo cp /path/to/source/file "/proc/$pid/root/"

Adapting to Kubernetes

The technique above is pretty low-level and needs some additional machinery to be adapted to Kubernetes.

Let’s recap our starting point in the previous section: we must be in the right host (i.e. the one where the container is running) and we know the identifier for the container we are after.

Let’s assume that we have our Pod name in variable pod, the namespace in variable ns, the container name in container. This is all we need to query our Kubernetes cluster for the additional information we are after.

Locating the host

First of all, let’s find out where the Pod is running (i.e. the worker node’s IP addres):

worker_ip="$(kubectl get -o jsonpath='{.status.hostIP}' \
             -n "$ns" "pod/$pod)"

Now that I notice it, this might be a wonderful alias or shell function:

alias worker_of="kubectl get -o jsonpath='{.status.hostIP}'"

Locating the container

Now let’s find out the container identifier:

# container identifier query
ciq="{.status.containerStatuses[?(.name == \"$container\")].containerID}"
cid="$(kubectl get -o jsonpath="$ciq" -n "$ns" "pod/$pod" \
       | sed -e 's#^docker://##')"

If you’re not sure what the target container is named like, you can print them all:

# container identifier list query
cilq='{range .status.containerStatuses[*]}{.name} {.containerID}{"\n"}{end}'
kubectl get -o jsonpath="$cilq" -n "$NAMESPACE" "pod/$POD_ID"
cid='...' # set from output of command above

Now we’re all set!

Conclusions

Putting a file in a container inside Kubernetes can be a daunting process if the container is not cooperating… but there are definitely ways to do this!