Certification Tip: Imperative Commands While you would be working mostly the declarative way - using definition files, imperative commands can help in getting one time tasks done quickly, as well as generate a definition template easily. This would help save a considerable amount of time during your exams.
Before we begin, familiarize with the two options that can come in handy while working with the below commands:
--dry-run: By default as soon as the command is run, the resource will be created. If you simply want to test your command, use the --dry-run=client option. This will not create the resource, instead, tell you whether the resource can be created and if your command is right.
-o yaml: This will output the resource definition in YAML format on the screen.
Use the above two in combination to generate a resource definition file quickly, that you can then modify and create resources as required, instead of creating the files from scratch.
POD Create an NGINX Pod
kubectl run nginx --image=nginx
Generate POD Manifest YAML file (-o yaml). Don't create it(--dry-run)
kubectl run nginx --image=nginx --dry-run=client -o yaml
Deployment Create a deployment
kubectl create deployment --image=nginx nginx =--------------------------------------------------------------------------= Certification Tip: Formatting Output with kubectl
The default output format for all kubectl commands is the human-readable plain-text format.
The -o flag allows us to output the details in several different formats.
kubectl [command] [TYPE] [NAME] -o <output_format>
Here are some of the commonly used formats:
-o jsonOutput a JSON formatted API object.
-o namePrint only the resource name and nothing else.
-o wideOutput in the plain-text format with any additional information.
-o yamlOutput a YAML formatted API object.
Here are some useful examples:
Output with JSON format:
master $ kubectl create namespace test-123 --dry-run -o json { "kind": "Namespace", "apiVersion": "v1", "metadata": { "name": "test-123", "creationTimestamp": null }, "spec": {}, "status": {} } master $
Output with YAML format:
master $ kubectl create namespace test-123 --dry-run -o yaml apiVersion: v1 kind: Namespace metadata: creationTimestamp: null name: test-123 spec: {} status: {}
Output with wide (additional details):
Probably the most common format used to print additional details about the object:
master $ kubectl get pods -o wide NAME READY STATUS RESTARTS AGE IP NODE NOMINATED NODE READINESS GATES busybox 1/1 Running 0 3m39s 10.36.0.2 node01 ningx 1/1 Running 0 7m32s 10.44.0.1 node03 redis 1/1 Running 0 3m59s 10.36.0.1 node01 master $
Generate Deployment YAML file (-o yaml). Don't create it(--dry-run)
kubectl create deployment --image=nginx nginx --dry-run=client -o yaml
IMPORTANT:
kubectl create deployment does not have a --replicas option. You could first create it and then scale it using the kubectl scale command.
Save it to a file - (If you need to modify or add some other details)
kubectl create deployment --image=nginx nginx --dry-run=client -o yaml > nginx-deployment.yaml
You can then update the YAML file with the replicas or any other field before creating the deployment.
Service Create a Service named redis-service of type ClusterIP to expose pod redis on port 6379
kubectl expose pod redis --port=6379 --name redis-service --dry-run=client -o yaml
(This will automatically use the pod's labels as selectors)
Or
kubectl create service clusterip redis --tcp=6379:6379 --dry-run=client -o yaml (This will not use the pods labels as selectors, instead it will assume selectors as app=redis. You cannot pass in selectors as an option. So it does not work very well if your pod has a different label set. So generate the file and modify the selectors before creating the service)
Create a Service named nginx of type NodePort to expose pod nginx's port 80 on port 30080 on the nodes:
kubectl expose pod nginx --port=80 --name nginx-service --type=NodePort --dry-run=client -o yaml
(This will automatically use the pod's labels as selectors, but you cannot specify the node port. You have to generate a definition file and then add the node port in manually before creating the service with the pod.)
Or
kubectl create service nodeport nginx --tcp=80:80 --node-port=30080 --dry-run=client -o yaml
(This will not use the pods labels as selectors)
Both the above commands have their own challenges. While one of it cannot accept a selector the other cannot accept a node port. I would recommend going with the kubectl expose command. If you need to specify a node port, generate a definition file using the same command and manually input the nodeport before creating the service.