Assigning an environment variable's value to a helm variable - ruby

Question

I'm new to helm and I want to be able to write gitlab project variables to files using config maps and shared environment variables.
I have a set of environment variables defined as gitlab project variables (the gitlab runner exposes them as environment variables) for each environment (where <ENV> is DEV/TEST/PROD for the sake of brevity):
MYSQL_USER_<ENV> = "user"
MYSQL_PASSWORD_<ENV> = "pass"
In the helm chart every environment has a map of its variables. For example, values.<ENV>.yaml contains:
envVars:
MYSQL_USER: $MYSQL_USER_<ENV>
MYSQL_PASSWORD: $MYSQL_PASSWORD_<ENV>
values.yaml contains a Ruby file which will consume those variables:
files:
config.rb: |
mysql['username'] = ENV["MYSQL_USER"]
mysql['password'] = ENV["MYSQL_PASSWORD"]
configmap.env.yaml defines:
apiVersion: v1
kind: ConfigMap
metadata:
name: {{ include "mychart.fullname" . }}-config-env
data:
{{- range $config_key, $config_value := .Values.envVars }}
{{ $config_key }}: {{ $config_value | quote | nindent 4 }}
{{- end }}
configmap.files.yaml defines:
apiVersion: v1
kind: ConfigMap
metadata:
name: {{ include "mychart.fullname" . }}-config-volume
data:
{{- range $file_path, $file_content := .Values.files }}
{{ $file_path }}: |
{{ $file_content | indent 4 -}}
{{- end }}
Finally, the deployment of the config map (only the config map part is shown here and I'm not using secrets here because this question is long enough as it is):
volumes:
- name: {{ include "mychart.fullname" . }}-config-volume
configMap:
name: {{ include "mychart.fullname" . }}-config-volume
containers:
- name: {{ .Chart.Name }}
volumeMounts:
- name: {{ include "mychart.fullname" . }}-config-volume
mountPath: /etc/my-config-dir
envFrom:
- configMapRef:
name: {{ include "mychart.fullname" . }}-config-env
So, in one sentence the workflow should be:
MYSQL_USER/PASSWORD_<ENV> saved into MYSQL_USER/PASSWORD, which are then written to /etc/my-config-dir/config.rb
I can't seem to make the environment variables of values.yaml (MYSQL_USER, MYSQL_PASSWORD) get the value of the project variables (MYSQL_USER_<ENV>, MYSQL_PASSWORD_<ENV>).
I use helm 3, but {{ env "MYSQL_USER_<ENV>" }} fails.
I could use string interpolation with the environment variable's name it the Ruby file, but then I would have to know what environment variables should be created for every container.
I'm trying to avoid having multiple --set arguments. Also I'm not sure how envsubst can be used here...
Any help will be greatly appreciated, thanks!

Answer 1

So eventually I used envsubst:
script:
- VALUES_FILE=values.${ENV}.yaml
- envsubst < ${VALUES_FILE}.env > ${VALUES_FILE}
- helm upgrade ... -f ${VALUES_FILE}