Kubernetes – kubectl command line utility

Hi All,

Greetings for the day 🙂 LIFE IS BEAUTIFUL 🙂

We are discussing Docker and Kubernetes. In previous few articles we discussed

• What is Kubernetes ? – https://knowledge-junction.in/2021/09/03/kubernetes-introduction-starting-point-for-beginners-helps-to-prepare-the-interview/
• Setting up local environment for Kubernetes – https://knowledge-junction.in/2021/09/11/kubernetes-simplifying-setting-up-local-environment-creating-kubernetes-cluster-locally-helps-in-interview-preparation-kubernetes-for-beginners/
• Yaml file guide / reference – https://knowledge-junction.in/2021/09/04/complete-guide-to-yaml/

Today we will discuss very important command line utility – kubectl

What is kubectl ?

• Command line utility / tool which communicate with Kubernetes cluster
• This utility allows us to run commands against Kubernetes cluster
• Note : If you want to know what is Kubernetes cluster please have a look once – https://knowledge-junction.in/2021/09/11/kubernetes-simplifying-setting-up-local-environment-creating-kubernetes-cluster-locally-helps-in-interview-preparation-kubernetes-for-beginners/
• We could install kubectl on Linux / Windows / macOS – please refer the above article in which we are discussing how to install kubectl on Linux

What exactly happen when kubectl command is get executed ?

• First understand the term Kubernetes API :
  • If we have a look below diagram, one of the core component of Kubernetes control plane is “kube API server”
  • API server exposes an HTTP API that allows users, different part of cluster and external components communicate with each other – we also called these API – Kubernetes API
  • Kubernetes API is an HTTP REST API
  • Kubernetes is fully controlled by these APIs
  • These Kubernetes API allows us to query and manipulate state of API objects in kubernetes like Pods, Namespaces, Configmaps and so on – kubernetes operations
  • All kubernetes operations exposes API end point
  • We can find API endpoint for all kubernetes in API references
  • So when we hit kubectl command kubectl makes HTTP POST request to respective operation API end point
• kubectl uses these Kubernetes API, most of the operations are performed by kubectl
• kubectl is CLI – command line interface which carries these HTTP API to the kubernetes API server as shown in below fig

Syntax for kubectl command:

kubectl [command] [TYPE] [NAME] [flags]

where,

• command
  • Operation which we want to perform
  • Example : create, get, describe, delete etc
• TYPE
  • Resource Type – Kubernetes objects
  • Example : pod, deployment, namespace, replicaset, ingress etc
  • To get all resource types we could execute CMDLET – kubectl api-resources
  • Resource types are case sensitive
  • we could specify singular , plural or abbreviated forms like pod / pods / po
• NAME
  • Name of the resource
  • Name is case-sensitive
• flags
  • Optional flags

Some important kubectl commands :

• To get the cluster information

kubectl cluster-info

• Create a following Kubernetes objects in default namespace :
  • Node
  • Pod
  • Replica Set
  • Deployment

kubectl create -f <Yammer definition file name of respective Kubernetes object>

Example:

kubectl create -f KnowledgeJunction-Pods-Definition-File.yaml

kubectl create -f KnowledgeJunction-ReplicaSets-Definition-File.yaml

kubectl create -f KnowledgeJunction-Depllyment-Definition-File.yaml

kubectl create -f KnowledgeJunction-Service-Definition-File.yaml

kubectl create -f KnowledgeJunction-Secret-Definition-File.yaml

• To create kubernetes object in another namespace, we need to use –namespace option

kubectl create -f <Yammer definition file name of respective Kubernetes object> --namespace=<namespace name>

• Get particular Kubernetes object lists from default namespace and hence the count

kubectl get <Kubernete Objects>

Example : 

kubectl get nodes
kubctl get pods
kubectl get replicasets
kubctl get deployments
kubectl get svc
kubectl get namespace
kubectl get secrets

• To verify multiple objects lists from default namespace in one command, seperate kubernetes objects by comma

kubectl get <Kubernetes Objects>, <Kubernetes Objects>

Example : 

kubectl get pods, svc

• To know the docker image used by Kubernetes object from default namespace use – -o wide option to kubctl get command

kubectl get <Kubernetes Objects>, <Kubernetes Objects> -o wide 

Example:

kubectl get pods
kubectl get pod pod1

• To see all the kubernete objects from default namespace created at a time use – kubectl get all

kubectl get all

• To list kubernetes object from another namespace use namespace option along with the name of namespace

kubectl get <Kubernetes Objects> -namespace=<namespace name>

• Get details of Kubernetes object using describe parameter

kubectl describe <kubernetes object> <object-name>

Example:

kubectl describe deployment knowledgejunction-deployment # get the details of deployment kubernetes object - knowledjejunction deployment

• Delete any kubernetes object

kubectl delete <objecttype> <objectname>

Example:

kubectl delete pod my-pod

• Delete all Kubernetes objects

kubectl delete <objecttype> --all

Example:

kubectl delete pods --all

• Check the status of Kubernetes Deployment object

kubectl rollout status deployment.apps/<deployment-name>

Example:

kubectl rollout status deployment.apps/knowledgeJunction-deployment

• To check / verify the history of deployments

kubectl rollout history deployment.apps/<deployment-name>

Example:

kubectl rollout history deployment.apps/knowledgeJunction-deployment

• To deploy application in cluster

kubectl run <application name> / <docker container>

Example:

kubectl run 

• To switch the default namespace permanently use => kubectl config command with namespace parameter having namespace name

kubectl config set-context $(kubectl config current-context) --namespace=dev

Thanks for reading ! HAVE A FANTASTIC TIME AHEAD !!