Kubernetes: Using kubectl.

In this article I would like to talk about using kubectl. “kubectl” as we mentioned is the CLI tool for Kubernetes which is a secure way to communicate with the Kube API server.

This means I can create, list, delete and update Kubernetes resources using "kubectl". In other words, we can apply HTTP methods on the restore API to the kube-api server in a simple and secure way!

I'll start with the basics all the way to advanced usage. An excellent list of commands is available here: https://kubernetes.io/docs/reference/generated/kubectl/kubectl-commands

Kubectl Create

Let's start by creating a resource:

kubectl create <ressourceType> <nomDeLaResource>

kubectl create deployment testdeploy --image=nginx

We can create it from a YAML file:

kubectl create -f demok8sresource.yaml

kubectl apply -f demok8sresource.yaml

Important Note: The difference between “create” and “apply” is that, “create” can only create a resource, while “apply” can create and update a resource.

For example, if the resource is already operational and you have made changes in the YAML, the “create” command will fail but “apply” will update the resource.

Create the resource and run the Pod directly

kubectl run <nomDuPod> --image=<image>

kubectl run nginx --image=nginx

List resources with Kubectl

To list any resource, we use “get”:

kubectl get <ressourceType>

kubectl get deployments

This will get all available deployments in the default namespace. For a specific namespace, we need to add the parameter “–namespace” or “-n” and specify the namespace name.

kubectl get pods -n default

To list pods in all namespaces:

kubectl get pods --all-namespaces

To list all development-related resources available in, you can use:

kubectl get all

This will give you most of the resources, but except for a few types like secrets etc.

We can get a specific resource:

kubectl get <ressourceType> <nomDeLaResource>

For more options, please refer here.

To get more information about a resource, we can describe it:

kubectl describe <ressourceType> <nomDeLaResource>

kubectl describe pod nginx

Advanced use of Kubectl

As we mentioned in previous series, kubectl sends an HTTP request and receives an HTTP response in JSON format. Kubectl embellishes this answer and gives us understandable output in a different format.

There are few different output options that you can use with '-o'.

For example:

kubectl get pods -o wide

As you can see, this will also give you the node and IP address information.

If we want to see the JSON output

kubectl get pods -o json

Since we have JSON output, we can get specific information about it using jsonpath.

kubectl get <ressourceType> -o jsonpath=’{jsonpath.to.the.information}’

kubectl get pods -o jsonpath=’{.items[*].metadata.name}’

This will return the pod names.

Another great option is “custom columns”. Using this flag we can beautify the output we want. As the name suggests, we create our custom columns and add the information that corresponds to them.

kubectl get <ressourceType> -o custom-columns=’<TITRE1>:<jsonpath>,<TITRE2>:<jsonpath>, …’

Important Note: When using jsonpath, you must mention the first item (i.e. .Items[]) because you are providing jsonpath. But when you use custom columns you ignore the first element.

kubectl get pods -o custom-columns=’NAME:.metadata.name’

Or with several columns:

kubectl get pods -o custom-columns='NAME:.metadata.name,NODE:.spec.name'

We can save the output in the format of our choice by adding > <FileName>.<fileType>:

kubectl get pods -o wide > pods.txt

kubectl get pods -o json > pods.json

or even custom columns:

kubectl get pods -o custom-columns=’NAME:.metadata.name,STATUS:.status.phase’ > podstatus.txt

I hope this article was useful to you. Thanks for reading it.

Find our #autourducode videos on our YouTube channel:https://bit.ly/3IwIK04