Kubernetes useful commands for CKAD exam

ckad.md

• Container commands:

  • Always use command: ['sh', '-c', 'command'] for safety.
  • Do not put your command and parameters in a single string: command: ['cowsay Welcome to CKAD!']
    • The whole string 'cowsay Welcome to CKAD!' is considered as a command which will throw error
  • Common bash script with loop:

    # writes output of command to a file
    while true; do date >> /opt/time/time-check.log; sleep 2 ;done

• Cron expressions:

  • Every minute: * * * * *
  • Every 30 minute: */30 * * * *
  • Every hour: 0 * * * *
  • Examples

• Logs:

  • View logs of a particular container: k logs my-pod -c my-container

• Grep:

  • [Logs] Match pattern with context('n' lines above and below):

    ✗ k logs nginx | grep OS --context=2
    2024/02/27 04:26:36 [notice] 1#1: nginx/1.25.4
    2024/02/27 04:26:36 [notice] 1#1: built by gcc 12.2.1 20220924 (Alpine 12.2.1_git20220924-r10)
    2024/02/27 04:26:36 [notice] 1#1: OS: Linux 6.6.12-linuxkit
    2024/02/27 04:26:36 [notice] 1#1: getrlimit(RLIMIT_NOFILE): 1048576:1048576
    2024/02/27 04:26:36 [notice] 1#1: start worker processes

• Test connectivity from another pod:

  • Netcat:

    # nc -vz <service-name> <port>
    nc -vz my-service-name 80

  • Curl

    # if curl is not present in alpine image, install using: apk add curl
    curl http://my-service:80

• Replace existing resource with force: k replace -f <file> --force

• Get JSON schema of a Custom Resource Definition(CRD)

  # search for openAPIV3Schema
  ✗ kubectl get crd crontabs.stable.example.com -o json

• List API Resources, group and version

  ✗ kubectl api-resources
  NAME                              SHORTNAMES   APIVERSION                             NAMESPACED   KIND
  bindings                                       v1                                     true         Binding
  componentstatuses                 cs           v1                                     false        ComponentStatus
  configmaps                        cm           v1                                     true         ConfigMap
  
  ✗ kubectl api-resources | grep configmaps

• List all resources, group and api version

  # view all nested fields recursively
  ✗ k explain pvc --recursive
  
  # explain specific property
  ✗ k explain pvc.spec.storageClassName --recursive

• Output custom columns from kubectl command:

  ✗ kubectl get pod --output=custom-columns="NAME:.metadata.name,QOS:.status.qosClass

• Helm

  Install a chart modifying some property e.g. replica count to 2

  • Find the chart and search the the field name:

    ✗ helm show values bitnami/apache | replica
    replicaCount: 1

  • Now we know the property name, we can modify it during install as:

    ✗ helm -n mercury install internal-issue-report-apache bitnami/apache --set replicaCount=2

  Other commands:

  # list repositories
  helm repo ls
  
  # list all repositories including pending
  helm repo ls -a
  
  # fetch latest versions of charts
  helm repo update
  
  # search a chart 'nginx' in all repositories
  helm search repo nginx
  
  # install a chart in a namespaace 'dev'
  helm -n dev install my-nginx-release bitnami/nginx 
  
  # delete a release and all it's resources
  helm uninstall my-nginx-release

• Get base64 decoded secret: k describe secret <secret-name>

• Vim

  • Save and exit wq!
  • Search: /
    • Case sensitive: /nodePort
    • Case insensitive:
      • \c will match anywhere in the string
      • E.g. /nodeport\c will match 'nodeport', 'NodePort' or other casings
    • Nagivate results: * for next occurrence # for prev
  • Line traversing skipping words
    • w: move forward to start of next word
    • b: move backward to start of prev word
  • Go to a line number: :<linenum> e.g. Goes to line 10: :10
  • Display line number: :set number
  • Jumping sentences/paragraphs:
    • (: prev sentence
    • ): next sentence