How to attach your GKE NodePort nginx-service to a precreated GLB

GLB.sh

#!/bin/bash
#set -x

## Predefined variables ##
#export GCP_CI_SVC_ACCOUNT=$(base64 my-svc-account-gcp.json)
#export GCP_CI_SVC_ACCOUNT_ID=my-svc-account-gcp-id@....google.com

export CLOUDSDK_CORE_DISABLE_PROMPTS=1
export CI_DOMAIN="ci.domain.tld"

export GCP_COMPUTE_ZONE="europe-west1-d"
export GCP_PROJECT_ID="ci-test-env"

export GKE_CLUSTER_NAME="php7-nginx-master-243"
export GKE_CLUSTER_FQDN="${GKE_CLUSTER_NAME}.${CI_DOMAIN}"

export GLB_NAME="ci-domain-tld"
export GLB_IP_NAME="glb01-ci-domain-tld"
export GLB_TLS_CERT="wildcardssl-ci-domain-tld"
export GLB_BACKEND_NAME="${GLB_NAME}-${GKE_CLUSTER_NAME}"
export GLB_HTTP_CHECK_NAME="${GKE_CLUSTER_NAME}-http-check"

## Evaluated variables ##

export GCP_REGION=$(echo "${GCP_COMPUTE_ZONE}" | sed -r 's#-\w+$##')
#export GLB_IP=$(gcloud compute addresses list --format="table[no-heading](address)" --filter="name=${GLB_IP_NAME}")
export LB_NODE_PORT=$(kubectl get --template="{{(index .spec.ports 0).nodePort}}" services nginx-service)
export GKE_GROUP_NAME=$(basename `gcloud beta container clusters describe ${GKE_CLUSTER_NAME} --zone ${GCP_COMPUTE_ZONE} | grep 'gke' | sed 's#.*/##' | sort -u | grep '^gke'`)
GKE_GROUP_SOME_NODE=$(gcloud compute instance-groups list-instances ${GKE_GROUP_NAME} --format="table[no-heading](instance)" | head -n1)
export GKE_NODE_TAG=$(gcloud compute instances describe --format="text[no-heading](tags)" ${GKE_GROUP_SOME_NODE} | awk '{print $2}' | grep '^gke-.*-node$')

# some outdated commands:
#gcloud compute url-maps create ${GLB_NAME} --default-service ${GLB_BACKEND_NAME}
#gcloud compute forwarding-rules create ${GLB_BACKEND_NAME}-http-rule --global --target-http-proxy ${GLB_BACKEND_NAME}-http-proxy --ports=80 --address ${GLB_IP_NAME}
#gcloud compute forwarding-rules create ${GLB_BACKEND_NAME}-https-rule --global --target-https-proxy ${GLB_BACKEND_NAME}-https-proxy --ports=443 --address ${GLB_IP_NAME}
#gcloud compute forwarding-rules list
#gcloud compute target-http-proxies create ${GLB_BACKEND_NAME}-http-proxy --url-map ${GLB_NAME}
#gcloud compute target-https-proxies create ${GLB_BACKEND_NAME}-https-proxy --ssl-certificates ${GLB_TLS_CERT} --url-map ${GLB_NAME}

# Suppose you have set up your env and auth for gcloud already
# if not, here are some useful commands:
#
#GCR_KEY_FILE="${HOME}/gcr-key.json"
#echo -n "${GCP_CI_SVC_ACCOUNT}" | base64 -d > "${GCR_KEY_FILE}"
#gcloud auth activate-service-account "${GCP_CI_SVC_ACCOUNT_ID}" --key-file "${GCR_KEY_FILE}"
#gcloud config set project "${GCP_PROJECT_ID}"
#gcloud config set compute/zone "${GCP_COMPUTE_ZONE}"
#gcloud config set container/cluster "${GKE_CLUSTER_NAME}"
#gcloud container clusters get-credentials "${GKE_CLUSTER_NAME}"

function add_to_glb() {
    # Allow connections to the exposed NodePort
	gcloud compute firewall-rules create ${GLB_BACKEND_NAME}-http-${LB_NODE_PORT} --allow tcp:${LB_NODE_PORT} --target-tags ${GKE_NODE_TAG}
    # Necessary HC to allow GLB to decide what nodes are healthy
	gcloud compute http-health-checks create ${GLB_HTTP_CHECK_NAME} --host "${GKE_CLUSTER_FQDN}" --port ${LB_NODE_PORT}	
	# Create a virtual backend, with details of target port, target instance group, autoscaling criterias and other mechanics
	gcloud compute backend-services create ${GLB_BACKEND_NAME} --protocol HTTP --port ${LB_NODE_PORT} --http-health-checks ${GLB_HTTP_CHECK_NAME} --global
	gcloud compute backend-services add-backend ${GLB_BACKEND_NAME} --balancing-mode UTILIZATION --max-utilization 0.8 --capacity-scaler 1 --instance-group ${GKE_GROUP_NAME} --instance-group-zone ${GCP_COMPUTE_ZONE} --global
    # Create the vhost on GLB and attach the virtual backend to it
	gcloud compute url-maps add-path-matcher ${GLB_NAME} --path-matcher-name "${GLB_BACKEND_NAME}-matcher" --default-service ${GLB_BACKEND_NAME} --new-hosts "${GKE_CLUSTER_FQDN}"
}

function del_from_glb() {		
	gcloud compute url-maps remove-path-matcher ${GLB_NAME} --path-matcher-name "${GLB_BACKEND_NAME}-matcher"	
	gcloud compute backend-services remove-backend ${GLB_BACKEND_NAME} --instance-group ${GKE_GROUP_NAME} --instance-group-zone ${GCP_COMPUTE_ZONE} --global
	gcloud compute backend-services delete ${GLB_BACKEND_NAME} --global
	gcloud compute http-health-checks delete ${GLB_HTTP_CHECK_NAME}
	gcloud compute firewall-rules delete ${GLB_BACKEND_NAME}-http-${LB_NODE_PORT}
}

case "$1" in
add)    add_to_glb
        ;;
delete) del_from_glb
        ;;
*)      echo "Usage: add|delete"
        exit 2
        ;;
esac