Introducing kubectl

Objectives

After completing this section, you should be able to review the basic usage of the kubectl command and understand how to connect to your Kubernetes cluster by using the CLI.

Introducing kubectl

kubectl is a Kubernetes command-line tool that allows you to interact with your Kubernetes cluster. It provides an easy way to perform tasks such as creating resources or redirecting cluster traffic. It is available for the three main operating systems (Linux, Windows and macOS).

For example, the following command displays the kubectl and Kubernetes version.

[user@host ~]$ kubectl version
Client Version: version.Info{Major:"1", Minor:"21", GitVersion:"v1.21.0", GitCommit:"cb303e613a121a29364f75cc67d3d580833a7479", GitTreeState:"clean", BuildDate:"2021-04-08T16:31:21Z", GoVersion:"go1.16.1", Compiler:"gc", Platform:"linux/amd64"}
Server Version: version.Info{Major:"1", Minor:"20", GpodsitVersion:"v1.20.0+75370d3", GitCommit:"75370d3fb99594d6f0263f3de0bd08237381b77d", GitTreeState:"clean", BuildDate:"2021-05-09T17:55:51Z", GoVersion:"go1.15.7", Compiler:"gc", Platform:"linux/amd64"}

Introducing kubectl configuration

Kubectl reads all the information necessary to connect to the Kubernetes cluster from a config file within your system. By default, this file is located at $HOME/kube/config. You can change this path by setting the environment variable KUBECONFIG to a custom file.

For example, the following sample sets the KUBECONFIG to the file /tmp/config.

[user@host ~]$ export KUBECONFIG=/tmp/config

All the commands related to the kubectl configuration are of the form:

kubectl config option

If you want to see what the configuration file contains, then you can use the following command.

[user@host ~]$ kubectl config view

The kubectl configuration file comprehends three topics:

  • Cluster: the URL for the API of a Kubernetes cluster. This URL identifies the cluster itself.

  • User: credentials that identify a user connecting to the Kubernetes cluster.

  • Context: puts together a cluster (the API URL) and a user (who is connecting to that cluster).

For example, you might have two contexts that are using different clusters but the same user.

Defining Clusters

It is often necessary to work with multiple clusters, so kubectl can hold the information of several Kubernetes clusters. In relation to the configuration for kubectl, a cluster is just the URL of the API of the Kubernetes cluster. The kubectl config set-cluster command allows you to create a new cluster connection by using the API URL.

For example, the following command creates a new cluster connection named my-cluster with server 127.0.0.1:8087.

[user@host ~]$ kubectl config set-cluster my-cluster --server=127.0.0.1:8087

Use the get-clusters option to list all available clusters.

[user@host ~]$ kubectl config get-clusters
my-cluster
my-cluster-2

Defining Users

The cluster configuration tells kubectl where the Kubernetes cluster is. The user configuration identifies who connects to the cluster. To connect to the cluster, it is necessary to provide an authentication method. There are several options to authenticate with the cluster:

  • Using a token

The following command creates a new user named my-user with the token Py93bt12mT.

[user@host ~]$ kubectl config set-credentials my-user --token=Py93bt12mT
  • Using basic authentication

The following command creates a new user named my-user with username kubernetes-username and password kubernetes-password.

[user@host ~]$ kubectl config set-credentials my-user --username=redhat-username --password=redhat-password
  • Using certificates

The following command creates a new user named my-user with a certificate redhat-certificate.crt and a key redhat-key.key.

[user@host ~]$ kubectl config set-credentials my-user --client-certificate=redhat-certificate.crt --client-key=redhat-key.key

Use the get-users option to list all available users.

[user@host ~]$ kubectl config get-users
my-user
my-user-2

Defining Contexts

A context puts together a cluster and a user. Kubectl uses both to connect and authenticate against a Kubernetes cluster.

For example, the following command creates a new context by using a cluster named my-cluster and a user named my-user.

[user@host ~]$ kubectl config set-context --cluster=my-cluster --user=my-user

In a kubectl context, it is possible to set a namespace. If provided, then any command would be executed in that namespace. The following command creates a context that points to the redhat-dev namespace.

[user@host ~]$ kubectl config set-context my-context --cluster=my-cluster --user=my-user --namespace=redhat-dev

Once a context has been created, you can select it by using the use-context command.

[user@host ~]$ kubectl config use-context my-context

After executing the previous command, further kubectl commands will use the my-cluster context and, therefore, the cluster and user associated to that context.

You can also list the contexts available in the configuration by using the get-contexts option. The * in the CURRENT column indicates the context that you are currently using.

[user@host ~]$ kubectl config get-contexts
CURRENT   NAME            CLUSTER           AUTHINFO      NAMESPACE
*         my-context      172.0.7.2:6443    my-user       redhat-dev
          my-context-2    172.1.8.0:6443    my-user-2

Another way of checking the current context is by using the current-context option.

[user@host ~]$ kubectl config current-context
my-context

Working with resources

Once you are connected to a Kubernetes cluster, kubectl allows you to list, create, update and delete Kubernetes resources. Most of these commands will be introduced in later chapters in the course, but there are some of them that can be mentioned at this point.

The get command

This commands is used to display one or more resources.

For example, kubectl get pods will display all pods in the current namespace.

[user@host ~]$ kubectl get pods
NAME       READY   STATUS        RESTARTS   AGE
example1   1/1     Running       0          67s
example2   1/1     Running       0          67s

If you want to display just the information for one pod, then add the pod’s name to the previous command.

[user@host ~]$ kubectl get pods example1
NAME       READY   STATUS        RESTARTS   AGE
example1   1/1     Running       0          67s

You can use this command to display other resources (services, jobs, ingresses…​).

Note

Use the command kubectl api-resources to display all resource types that you can create.

The delete command

This command allows you to delete a resource.

For example, kubectl delete pod example1 deletes the pod named example1.

[user@host ~]$ kubectl delete pod example1
pod "example1" deleted

You can use this command to delete other resources (services, jobs, ingresses…​).

The apply command

A common way to manage Kubernetes resources is by using a manifest. A manifest is a YML or JSON file containing one or many Kubernetes resources.

The apply command allows you to create, update or delete resources from a manifest.

For example, the following snippet creates a Deployment resource.

apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
  labels:
    app: nginx
spec:
  replicas: 3
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.7.9
        ports:
        - containerPort: 80

If the snippet was in a file named deployment.yml, then you could use apply to create the deployment. Note that the -f option is used to indicate the file.

[user@host ~]$ kubectl apply -f deployment.yml
deployment.apps/nginx-deployment created

 

Tags:

kubectl