Parsing Kubernetes ConfigMap in Mixed Data Formats - json

Question

Consider this current namespace config in JSON format:
$ kubectl get configmap config -n metallb-system -o json
{
"apiVersion": "v1",
"data": {
"config": "address-pools:\n- name: default\n protocol: layer2\n addresses:\n - 192.168.0.105-192.168.0.105\n - 192.168.0.110-192.168.0.111\n"
},
"kind": "ConfigMap",
"metadata": {
"annotations": {
"kubectl.kubernetes.io/last-applied-configuration": "{\"apiVersion\":\"v1\",\"data\":{\"config\":\"address-pools:\\n- name: default\\n protocol: layer2\\n addresses:\\n - 192.168.0.105-192.168.0.105\\n - 192.168.0.110-192.168.0.111\\n\"},\"kind\":\"ConfigMap\",\"metadata\":{\"annotations\":{},\"name\":\"config\",\"namespace\":\"metallb-system\"}}\n"
},
"creationTimestamp": "2020-07-10T08:26:21Z",
"managedFields": [
{
"apiVersion": "v1",
"fieldsType": "FieldsV1",
"fieldsV1": {
"f:data": {
".": {},
"f:config": {}
},
"f:metadata": {
"f:annotations": {
".": {},
"f:kubectl.kubernetes.io/last-applied-configuration": {}
}
}
},
"manager": "kubectl",
"operation": "Update",
"time": "2020-07-10T08:26:21Z"
}
],
"name": "config",
"namespace": "metallb-system",
"resourceVersion": "2086",
"selfLink": "/api/v1/namespaces/metallb-system/configmaps/config",
"uid": "c2cfd2d2-866c-466e-aa2a-f3f7ef4837ed"
}
}
I am interested only in the address pools that are configured. As per the kubectl cheat sheet, I can do something like this to fetch the required address range:
$ kubectl get configmap config -n metallb-system -o jsonpath='{.data.config}'
address-pools:
- name: default
protocol: layer2
addresses:
- 192.168.0.105-192.168.0.105
- 192.168.0.110-192.168.0.111
However, my requirement is to use only a JSON parser throughout, and I cannot parse the above output since it is in YAML instead.
Since I'm not willing to accomodate the above yaml output for direct use ( or via format conversion operation ), is there any suitable way I can obtain the address range from the kubectl interface in a JSON format instead?

Answer 1

You need yq alongside kubectl ...
After inspecting your configmap, I understood the structure when I convert it to YAML :
---
apiVersion: v1
data:
config: |
address-pools:
- name: default
protocol: layer2
addresses:
- 192.168.0.105-192.168.0.105
- 192.168.0.110-192.168.0.111
kind: ConfigMap
metadata:
name: config
And you can see clearly that .data.config is a multi-line string.. but it can be converted also to YAML.
parse the multi-line string with kubectl
treat this string as yaml using yq.
So This is what you are looking for :
# all addresses
kubectl -n metallb-system get cm config -o 'go-template={{index .data "config" }}' | \
yq -r '.["address-pools"][0].addresses'
# first address only
kubectl -n metallb-system get cm config -o 'go-template={{index .data "config" }}' | \
yq -r '.["address-pools"][0].addresses[0]'
# second address only
kubectl -n metallb-system get cm config -o 'go-template={{index .data "config" }}' | \
yq -r '.["address-pools"][0].addresses[1]'
# so on ...