How Can You Perform Variable Substitution using Kubectl?

Question

I am trying to create a Role and RoleBinding so I can use Helm. I want to use variable substitution somehow to replace {{namespace}} with something when I run an apply command.

kind: Role
apiVersion: rbac.authorization.k8s.io/v1
metadata:
  name: tiller-manager-{{namespace}}
  namespace: {{namespace}}
rules:
- apiGroups: ["", "batch", "extensions", "apps"]
  resources: ["*"]
  verbs: ["*"]

I want to pass the namespace something like this:

kubectl apply --file role.yaml --namespace foo

I have seen that kubectl apply has a --template parameter but I can't see much information about how it might be used.

For what you’ve shown here, you don’t need to specify the namespace in the YAML file: `kubectl` has a `--namespace` option to fill that item in, and since resource names are namespace-scoped, you can have the same role name in multiple namespaces safely. — David Maze, Nov 13 '18 at 18:53

score 5 · Accepted Answer · answered Nov 13 '18 at 17:00

5

You can do it in following way.

Write Role file like this:

kind: Role
apiVersion: rbac.authorization.k8s.io/v1
metadata:
  name: tiller-manager-${NAMESPACE}
  namespace: ${NAMESPACE}
rules:
- apiGroups: ["", "batch", "extensions", "apps"]
  resources: ["*"]
  verbs: ["*"]

Set NAMESPACE environment variable to your desired value.

Then create Role using following command

envsubst < role.yaml | kubectl apply -f -

answered Nov 13 '18 at 17:00

Emruz Hossain

4,764
18
26

I guess I have to install `envsubst` though first. – Muhammad Rehan Saeed Nov 13 '18 at 17:04
If you are using Mac check answer from this: https://stackoverflow.com/questions/23620827/envsubst-command-not-found-on-mac-os-x-10-8 – Emruz Hossain Nov 13 '18 at 17:09

score 1 · Answer 2 · answered Nov 15 '18 at 06:40

Another way you can use. With sed command you can replace {{namespace}} directly. without setting any environment variable or using envsubst.

First save your role in a file say role.yaml which contains original content.
Then use sed 's/{{namespace}}/your-namespace-name/g' role.yaml. which print in stdout the desired file. Replaced {{namespace}} with your-namespace-name.

N.B: you can also save it in a file using sed 's/{{namespace}}/your-namespace-name/g' role.yaml > new-role.yaml

score 1 · Answer 3 · edited Oct 13 '20 at 11:45

1

Can be done like this.

cat .env
IMAGE_SOURCE=hello
IMAGE_VERSION=2.3.4

source .env; kustomize build dev | envsubst

edited Oct 13 '20 at 11:45

4b0

21,981
30
95
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answered Oct 13 '20 at 11:39

Lokesh Kumar

51
1

1

source .env; kustomize build dev | envsubst | kubectl create -f - – Lokesh Kumar Oct 13 '20 at 11:42

How Can You Perform Variable Substitution using Kubectl?

3 Answers3