How to create a kubectl config file for serviceaccount

# your server name goes here
server=https://localhost:8443
# the name of the secret containing the service account token goes here
name=default-token-sg96k

ca=$(kubectl get secret/$name -o jsonpath='{.data.ca\.crt}')
token=$(kubectl get secret/$name -o jsonpath='{.data.token}' | base64 --decode)
namespace=$(kubectl get secret/$name -o jsonpath='{.data.namespace}' | base64 --decode)

echo "
apiVersion: v1
kind: Config
clusters:
- name: default-cluster
  cluster:
    certificate-authority-data: ${ca}
    server: ${server}
contexts:
- name: default-context
  context:
    cluster: default-cluster
    namespace: default
    user: default-user
current-context: default-context
users:
- name: default-user
  user:
    token: ${token}
" > sa.kubeconfig

I cleaned up Jordan Liggitt's script a little.

Unfortunately I am not yet allowed to comment so this is an extra answer:

# The script returns a kubeconfig for the service account given
# you need to have kubectl on PATH with the context set to the cluster you want to create the config for

# Cosmetics for the created config
clusterName=some-cluster
# your server address goes here get it via `kubectl cluster-info`
server=https://157.90.17.72:6443
# the Namespace and ServiceAccount name that is used for the config
namespace=kube-system
serviceAccount=developer

######################
# actual script starts
set -o errexit

secretName=$(kubectl --namespace $namespace get serviceAccount $serviceAccount -o jsonpath='{.secrets[0].name}')
ca=$(kubectl --namespace $namespace get secret/$secretName -o jsonpath='{.data.ca\.crt}')
token=$(kubectl --namespace $namespace get secret/$secretName -o jsonpath='{.data.token}' | base64 --decode)

echo "
---
apiVersion: v1
kind: Config
clusters:
  - name: ${clusterName}
    cluster:
      certificate-authority-data: ${ca}
      server: ${server}
contexts:
  - name: ${serviceAccount}@${clusterName}
    context:
      cluster: ${clusterName}
      namespace: ${namespace}
      user: ${serviceAccount}
users:
  - name: ${serviceAccount}
    user:
      token: ${token}
current-context: ${serviceAccount}@${clusterName}
"

Kubectl can be initialized to use a cluster account. To do so, get the cluster url, cluster certificate and account token.

KUBE_API_EP='URL+PORT'
KUBE_API_TOKEN='TOKEN'
KUBE_CERT='REDACTED'

echo $KUBE_CERT >deploy.crt
kubectl config set-cluster k8s --server=https://$KUBE_API_EP \ 
    --certificate-authority=deploy.crt  \
    --embed-certs=true
kubectl config set-credentials gitlab-deployer --token=$KUBE_API_TOKEN
kubectl config set-context k8s --cluster k8s --user gitlab-deployer
kubectl config use-context k8s

The cluster file is stored under: ~/.kube/config. Now the cluster can be accessed using:

kubectl --context=k8s get pods -n test-namespace

add this flag --insecure-skip-tls-verify if you are using self signed certificate.