TL;DR

Can I reference a previously-defined key-value pair in a ConfigMap?

Full Version

I'm writing a deployment specification for an application which accepts
its configuration from environment variables at startup. Some of the
environment variables are derived from others. If I were setting
them up as file to be sourced at application startup, I would simply
do:

[me@myserver ~]$ cat ./myenv.sh
export FOO=foo
export BAR=bar
export FOOBAR=$FOO$BAR
[me@myserver ~]$ . ./myenv.sh
[me@myserver ~]$ printenv FOOBAR
foobar

However, the analagous way to do this in a ConfigMap by referencing
previously-defined key-value pairs doesn't work (see sample ConfigMap
and Pod, below). Here are the results:

[me@myserver ~]$ kubectl create -f my-app-config.yaml -f my-app-pod.yaml
configmap "my-app" created
pod "my-app" created
[me@myserver ~]$ kubectl exec -it my-app -- printenv | grep MY_CONFIGMAP
MY_CONFIGMAP_FOO=foo
MY_CONFIGMAP_FOOBAR=$(MY_CONFIGMAP_FOO)$(MY_CONFIGMAP_BAR)
MY_CONFIGMAP_BAR=bar

Hoped-for value of MY_CONFIGMAP_FOOBAR is foobar.

I attempted to cross-apply the following instructions under
Use ConfigMap-defined environment variables in Pod commands
from the documentation:

You can use ConfigMap-defined environment variables in the command
section of the Pod specification using the $(VAR_NAME)
Kubernetes substitution syntax.

However, this obviously did not work, and I've been unable to find an
answer in my research.

I suspect I already know the answer, but is there any way to accomplish
this?

MCVE

ConfigMap

apiVersion: v1
kind: ConfigMap
metadata:
 name: my-app

data:
 MY_CONFIGMAP_FOO: foo
 MY_CONFIGMAP_BAR: bar

 # Desired concatenation would be `foobar`
 MY_CONFIGMAP_FOOBAR: $(MY_CONFIGMAP_FOO)$(MY_CONFIGMAP_BAR)

Pod

apiVersion: v1
kind: Pod
metadata:
 name: my-app

spec:
 containers:
 - name: my-app
 image: alpine
 imagePullPolicy: IfNotPresent
 envFrom:
 - configMapRef:
 name: my-app

 command:
 - /bin/sh

 args:
 - -c
 - "while true; do sleep 3600; done"

You can achieve your expected result with help of this tool: envsubst.

Create the ConfigMap following way,

envsubst < my-app-config.yaml | kubectl apply -f -

Now, all environment variables on your ConfigMap will be replaced with resolved value.