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Command line tool kubectl for kubernetes learning

Posted on Aug. 8, 2022, 4:55 p.m. by Richard Maxwell
Category: The back-end Tag: kubernetes

An overview of the

Kubectl is the command line tool of K8S cluster, through Kubectl can manage the cluster itself, and can install and deploy containerization applications on the cluster.

kubectl [command] [type] [name] [flags]
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  1. commad: Specifies the operations to be performed on the resource, such as create, GET, Describe, and DELETE
  2. type: Specifies the resource type. The resource type is case sensitive.
  3. name: Specifies the name of the resource, which is also case-sensitive
  4. flags: Specifies optional parameters. For example, the -s or -server arguments can be used to specify the address and port of the Kubernetes API Server.

Kubectl main command

  1. Basic commands
operation instructions
create Create resources using file names or standard input
expose Expose a resource as a new Service
run Run a specific image in the cluster
set Set specific functions on objects
get Displays one or more resources
explain Documentation References
edit Edit a resource using the default editor
delete Delete a resource by filename, standard input, resource name, or label selector

2. Deploy commands

operation instructions
rollout Manage the release of resources
rollout-update Roll updates to the given replication controller
scale Expand or reduce the number of PODS, Deployment, ReplicaSet, RC, or Jobs
autoscale Create an automatic capacity expansion or reduction option and set the number of PODS
  1. Cluster Management Commands
operation instructions
certificate Modifying certificate Resources
cluster-info Displaying Cluster Information
top Display resources (CPU/Memory/Storage) use, need to run Heapster
cordon Flag the node is not schedulable
uncordon Tag nodes are schedulable
drain Delete applications from the node and prepare for offline maintenance
taint Modify the node taint flag

4. Fault diagnosis and debugging commands

operation instructions
describe Displays detailed information about a particular resource or resource group
logs Print a container log in a POD. If the POD has only one container, the container name is optional
attach Attach to a running container
exec Execute commands to containers
port-forward Forward one or more local ports to a POD
proxy Run a proxy into Kubernetes API Server
cp Copy files or directories to containers
auth Check the authorization
  1. Other commands
operation instructions
apply Apply configuration to resources by file name or standard input
patch Use patches to modify and update resource fields
replace Replace a resource with a filename or standard input
convert Convert configuration files between different API versions
label Update labels on resources
annotate Update comments on resources
completion Used to achieve kubectl command automatic completion
api-versions Prints supported API versions
config Kubeconfig file (for accessing apis, such as configuring authentication information)
help Print command help
plugin Run a command line plug-in
version Print client and service version information

Gets the specific syntax of the command

kubectl command --help
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Gets all resource types currently supported by the server

kubectl api-resources
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Prints all the optional flags currently supported

kubectl options
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For example, -s can specify the Kubernetes API server address, -n can specify the namespace, --kubeconfig can specify the Kubeconfig configuration file

(Figure source network)

Format output command

By default, the default output format of all Kubectl commands is readable plain text. To output details to a terminal window in a specific format, we need to use "-o" or multiple "-output" flags.

Command format:

kubectl [command] [TYPE] [NAME] -o=output_format
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Common output formats

The output format describe
-o=custom-columns=spec Enter the specified comma-separated list of column names to print the table.
-o=custom-columns-file=filename Use the custom column template in the file to print the table.
-o=json Outputs an API object in JSON format.
-o=jsonpath=template Print the fields defined in the JsonPath expression
-o=jsonpath-file=filename Prints the fields defined by the JSONPath expression in the file.
-o=name Only the resource name is printed.
-o=wide Output any additional information in plain text format. For pods, include node names.
-o=yaml Output YAML API objects.

Examples of Common Commands

  1. Create a resource
#According to the YAML file to create the corresponding resource, with the record parameter will record the update history, can be added or not added
kubectl apply -f nginx.yaml --record
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  1. See the resources
#View resources in the specified namespace
kubectl get pod -n default
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#Viewing All Resources
kubectl get pod -A
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#Viewing POD Details
kubectl get pod nginxdeployment-585449566-h6hng -n default -o wide
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  1. Delete the resource

kubectl delete pod nginxdeployment-585449566-h6hng -n default -o wide
kubectl delete -f nginx.yaml

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  1. Manually expand or shrink the capacity
#If the replicas is larger than the current NUMBER of PODS, it is expanded; if the replicas is smaller, it is reduced
 kubectl scale deployment nginxdeployment --replicas=4
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  1. The rollback
#Rollback (back to the last revision in the record)
kubectl rollout undo deployment nginxdeployment
#See the history
kubectl rollout history deployment nginxdeployment
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6. Edit and update resources

#Use the default editor to edit the corresponding resource file. After saving the file, the resource attributes will be updated
kubectl edit -f nginx.yaml
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The original two pods were changed to one pod by modifying the nginx.yaml file

Everyday the debug

Take pod resources as an example

  1. Check whether the corresponding POD exists first. If yes, check whether the status is normal

kubectl get pod coredns-f9fd979d6-kvpmn -n kube-system -o wide
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  1. View details about the POD that needs to be rectified. Note that namespace must be followed

3. If the cause cannot be found in the previous step, print related logs

#Refresh print logs in real time, with namespace followed by n and podName
 kubectl logs -f -n kube-system etcd-minikube
#Displays the last 50 lines of the log
kubectl logs --tail=50 -n kube-system etcd-minikube
#Displays logs generated in the past 2h
kubectl logs --since=2h -n kube-system etcd-minikube
#Specifying a configuration file
kubectl --kubeconfig="log-watcher.kubeconfig" logs -f etcd-minikube -n kube-system
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  1. Unable to solve the problem, go inside the container to find the cause
   kubectl exec -it -n kube-system etcd-minikube -- sh
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