how to merge kubeconfig files [k8s]

script.sh

cd $(mktemp -d)

# `./config` - existent kubeconfig to merge to (like ~/.kube/config)
# `./configToAdd` - new kubeconfig to merge from

cat <<_EOF_ > ./config
apiVersion: v1
clusters:
- cluster:
    certificate-authority-data: MQo=
    server: https://K8S-DOMAIN-NAME-1
  name: CLUSTER-1
- cluster:
    certificate-authority-data: Mgo=
    server: https://K8S-DOMAIN-NAME-2
  name: CLUSTER-2
contexts:
- context:
    cluster: CLUSTER-1
    namespace: default
    user: USER-1
  name: CONTEXT-1
- context:
    cluster: CLUSTER-2
    namespace: default
    user: USER-2
  name: CONTEXT-2
current-context: CONTEXT-1
kind: Config
preferences: {}
users:
- name: USER-1
  user:
    exec:
      args: null
      command: aws
      env: null
      provideClusterInfo: false
- name: USER-2
  user:
    exec:
      args: null
      command: aws
      env: null
      provideClusterInfo: false
_EOF_

cat <<_EOF_ > ./configToAdd
apiVersion: v1
clusters:
- cluster:
    certificate-authority-data: Mwo=
    server: https://K8S-DOMAIN-NAME-3
  name: CLUSTER-3
contexts:
- context:
    cluster: CLUSTER-3
    namespace: default
    user: USER-3
  name: CONTEXT-3
current-context: CONTEXT-3
kind: Config
preferences: {}
users:
- name: USER-3
  user:
    exec:
      args: null
      command: aws
      env: null
      provideClusterInfo: false
_EOF_

KUBECONFIG=./config:./configToAdd kubectl config view --flatten | sponge ./config

# `./config` now contains both initial and merged data