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explore-kubectl.md

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Explore kubectl

Get Started

kubectl, said Kube-Control, is the command line interface for running commands against a Kubernetes clusters. In this execise, you will explore some useful features of kubectl that you may find useful in the CKAD exam.

  1. Get Cluster information.

    kubectl cluster-info

  2. Write Cluster information to a file name 'cluster.txt'.

    kubectl cluster-info > cluster.txt

  3. Get a list of nodes in the cluster.

    kubectl get nodes

  4. Get a list of all namespaces in the cluster.

    kubectl get namespaces

  5. Get a list of all pods, including their IP addresses, running in the cluster.

    kubectl get pods --all-namespaces -o wide

    Note: This instruction says "all" and "including their IP addresses". Watch out for key words like this in the CKAD exam questions. For example, kubectl get pods would be incorrect.

  6. Find out CPU and memory limits for the first node in the cluster

    # get list of nodes
kubectl get nodes

# using the first one, describe the node
kubectl describe node <node-name>

    or

    # once you become more familar with jsonpath, you could do the same like this
kubectl get nodes -o jsonpath='{.items[0].metadata.name}' | kubectl describe node

Generate yaml

Writing yaml files quickly is an essential skill in the CKAD exam. In a lot of cases you can use kubectl to generate yaml for you.

Make sure you understand the constructs and syntax of yaml: www.json2yaml.com

  1. Generate the yaml to create a namespace called 'new-namespace'.

    kubectl create namespace new-namespace --dry-run -o yaml

    Note: the use of --dry-run -o yaml to generate the yaml.

    • --dry-run - prints the object that would be sent, without sending it.
    • -o yaml - changes the output format to yaml

  2. Write the yaml to create a namespace called 'new-namespace' to a file named 'namespace.yaml'.

    # write the generate yaml to disk
kubectl create namespace new-namespace --dry-run -o yaml > namespace.yaml

# check the contents of the file
cat namespace.yaml

Export

Exported resources are stripped of cluster-specific information. This option is useful to obtain the yaml to recreate a particular object.

  1. Get a pod nginx in namespace demospace without any cluster specific information

    kubectl get pod nginx -n demospace --export -o yaml

Explain

kubectl explain will explain the given resource. For example, a top level API-object like Pod or a specific field like a Pod's container.

  1. Get the documentation of a Pod resource and its fields.

    kubectl explain pods

  2. Get the documentation for a Pod's container specification.

    kubectl explain pods.spec.containers

    Note: pods.spec.containers matches the yaml object structure:

    apiVersion: v1
kind: Pod
metadata:
creationTimestamp: null
name: nginx
spec:
  containers:
  - image: nginx
    imagePullPolicy: IfNotPresent
    name: nginx
    resources: {}
  dnsPolicy: ClusterFirst
  restartPolicy: Never