Set values in a knative service.yaml file using environment variables - kubernetes

Question

Is there a way to set the values of some keys in a Knative service.yaml file using environment variables?
More detail
I am trying to deploy a Knative service to a Kubernetes cluster using GitLab CI. Some of the variables in my service.yaml file depend on the project and environment of the GitLab CI pipeline. Is there a way I can seamlessly plug those values into my service.yaml file without resorting to hacks like sed -i ...?
For example, given the following script, I want the $(KUBE_NAMESPACE), $(CI_ENVIRONMENT_SLUG), and $(CI_PROJECT_PATH_SLUG) values to be replaced by accordingly-named environment variables.
apiVersion: serving.knative.dev/v1
kind: Service
metadata:
name: design
namespace: "$(KUBE_NAMESPACE)"
spec:
template:
metadata:
name: design-v1
annotations:
app.gitlab.com/env: "$(CI_ENVIRONMENT_SLUG)"
app.gitlab.com/app: "$(CI_PROJECT_PATH_SLUG)"
spec:
containers:
- name: user-container
image: ...
timeoutSeconds: 600
containerConcurrency: 8

Answer 1

I don't think there is a great way to expand environment variables inside of an existing yaml, but if you don't want to use sed, you might be able to use envsubst:
envsubst < original.yaml > modified.yaml
You would just run this command before you use the yaml to expand the environment variables contained within it.
Also I think you'll need your variables to use curly braces, instead of parentheses, like this: ${KUBE_NAMESPACE}.
EDIT: You might also be able to use this inline like this: kubectl apply -f <(envsubst < service.yaml)

Answer 2

More than a Knative issue this is more of Kubernetes limitation. Kubernetes allows some expansion but not in annotations or namespace definitions. For example, you can do it in container env definitions:
containers:
- env:
- name: PODID
valueFrom: ...
- name: LOG_PATH
value: /var/log/$(PODID)
If this is a CI/CD system like Gitlab the environment variables should be in a shell environment, so a simple shell expansion will do. For example.
#!/bin/bash
echo -e "
apiVersion: serving.knative.dev/v1
kind: Service
metadata:
name: design
namespace: "${KUBE_NAMESPACE}"
spec:
template:
metadata:
name: design-v1
annotations:
app.gitlab.com/env: "${CI_ENVIRONMENT_SLUG}"
app.gitlab.com/app: "${CI_PROJECT_PATH_SLUG}"
spec:
containers:
- name: user-container
image: ...
timeoutSeconds: 600
containerConcurrency: 8
" | kubectl apply -f -
You can also use envsubst as a helper like mentioned in the other answer.