useful_commands

Useful Commands

Objective:
This section details some useful commands and handy tips.

A. BASH

When you get more & more familiar with Kubernetes, you start wondering how to be more efficient with typing the commands... One option would to create a bunch of alias in the .bashrc file.

For example:

# cat <<EOT >> ~/.bashrc
source <(kubectl completion bash)
alias k=kubectl
complete -F __start_kubectl k

alias kc='kubectl create'
alias kg='kubectl get'
alias kdel='kubectl delete'
alias kdesc='kubectl describe'
alias kedit='kubectl edit'
EOT

Don't forget to type in bash in order to take the modifications into account.

B. Kubectl Autocomplete

Kubectl has support for auto-completion allowing you to discover the available options. This is applied to the terminal session using the source command:

source <(kubectl completion bash) # setup autocomplete in bash into the current shell, bash-completion package should be installed first.
echo "source <(kubectl completion bash)" >> ~/.bashrc # add autocomplete permanently to your bash shell.

C. Basic and troubleshooting commands

Below are a number of kubectl commands that might come handy in general, viewing additional resource information or assist in troubleshooting various scenarios:

kubectl get

Prints a table of the most important information about the specified resources. You can filter the list using a label selector and the --selector flag. If the desired resource type is namespaced you will only see results in your current namespace unless you pass --all-namespaces (shorthand -A) or specify the namespace using --namespace specific-namespace (shorthand -n specific-namespace).

Usage:
  kubectl get
[(-o|--output=)json|yaml|wide|custom-columns=...|custom-columns-file=...|go-template=...|go-template-file=...|jsonpath=...|jsonpath-file=...]
(TYPE[.VERSION][.GROUP] [NAME | -l label] | TYPE[.VERSION][.GROUP]/NAME ...) [flags] [options]

Example:
# List all objects in the default namespace
[root@rhel3 ~]# kubectl get all

kubectl describe

Print a detailed description of the selected resources, including related resources such as events or controllers. You may select a single object by name, all objects of that type, provide a name prefix, or label selector.

Use "kubectl api-resources" for a complete list of supported resources.

Usage:
  kubectl describe (-f FILENAME | TYPE [NAME_PREFIX | -l label] | TYPE/NAME) [options]

Example:
# Describe a pod
[root@rhel3 ~]# kubectl describe pods trident-csi-788b4d865c-xdzn7 -n trident

kubectl logs

Print the logs for a container in a pod or specified resource. If the pod has only one container, the container name is optional.

Usage:
  kubectl logs [-f] [-p] (POD | TYPE/NAME) [-c CONTAINER] [options]

Example:
# Return snapshot logs from container trident-main of a deployment named  deployment.apps/trident-csi
[root@rhel3 ~]# kubectl logs trident-csi-788b4d865c-xdzn7 -c trident-main -n trident

kubectl events

Report of an event somewhere in the cluster.

Usage:
  kubectl get
[(-o|--output=)json|yaml|wide|custom-columns=...|custom-columns-file=...|go-template=...|go-template-file=...|jsonpath=...|jsonpath-file=...]
(TYPE[.VERSION][.GROUP] [NAME | -l label] | TYPE[.VERSION][.GROUP]/NAME ...) [flags] [options]

Example:
# Get events for namespace trident
[root@rhel3 ~]# kubectl get events -n trident

kubectl run

Create and run a particular image in a pod.

Usage:
  kubectl run NAME --image=image [--env="key=value"] [--port=port] [--dry-run=server|client] [--overrides=inline-json]
[--command] -- [COMMAND] [args...] [options]

Example:
# Run a shell in an interactive pod - very useful for debugging
[root@rhel3 ~]# kubectl run -i --tty busybox --image=busybox --restart=Never -- sh

kubectl exec

Execute a command in a container.

Usage:
  kubectl exec (POD | TYPE/NAME) [-c CONTAINER] [flags] -- COMMAND [args...] [options]

Example:
# Verify that the volume is mounted on /usr/share/nginx/html
[root@rhel3 ~]# kubectl exec -it nginx -- df -h /usr/share/nginx/html

kubectl attach

Attach to a process that is already running inside an existing container.

Usage:
  kubectl attach (POD | TYPE/NAME) -c CONTAINER [options]

Example:
# Attach to a running pod called nginx
[root@rhel3 ~]# kubectl attach nginx -it

If you need any further assistance, feel free to ask your bootcamp host.

D. How can I easily list all the containers in a POD

As there is no option to do so, you need to extract this information from the POD definition. Below is an example output with Trident Operator 20.04 and Kubernetes 1.18:

[root@rhel3 ~]# kubectl get pods -n trident -o=jsonpath='{range .items[*]}{"\n"}{.metadata.name}{":\t"}{range .spec.containers[*]}{.image}{", "}{end}{end}' |sort

trident-csi-788b4d865c-xdzn7:   netapp/trident:20.04, quay.io/k8scsi/csi-provisioner:v1.6.0, quay.io/k8scsi/csi-attacher:v2.2.0, quay.io/k8scsi/csi-resizer:v0.5.0, quay.io/k8scsi/csi-snapshotter:v2.1.0,
trident-csi-gn4cv:      netapp/trident:20.04, quay.io/k8scsi/csi-node-driver-registrar:v1.3.0,
trident-csi-q2w6c:      netapp/trident:20.04, quay.io/k8scsi/csi-node-driver-registrar:v1.3.0,
trident-csi-rdpjk:      netapp/trident:20.04, quay.io/k8scsi/csi-node-driver-registrar:v1.3.0,
trident-csi-x8ppp:      netapp/trident:20.04, quay.io/k8scsi/csi-node-driver-registrar:v1.3.0,
trident-operator-668bf8cdff-577p9:      netapp/trident-operator:20.04.0,

Below is an example output with Trident Operator 20.07 and Kubernetes 1.18:

[root@rhel3 ghost]# kubectl get pods -n trident -o=jsonpath='{range .items[*]}{"\n"}{.metadata.name}{":\t"}{range .spec.containers[*]}{.image}{", "}{end}{end}' |sort

trident-csi-7bb4bfb84-n2gnz:    netapp/trident:20.07.0, netapp/trident-autosupport:20.07.0, quay.io/k8scsi/csi-provisioner:v1.6.0, quay.io/k8scsi/csi-attacher:v2.2.0, quay.io/k8scsi/csi-resizer:v0.5.0, quay.io/k8scsi/csi-snapshotter:v2.1.1,
trident-csi-g2bq9:      netapp/trident:20.07.0, quay.io/k8scsi/csi-node-driver-registrar:v1.3.0,
trident-csi-nkmhj:      netapp/trident:20.07.0, quay.io/k8scsi/csi-node-driver-registrar:v1.3.0,
trident-csi-q6646:      netapp/trident:20.07.0, quay.io/k8scsi/csi-node-driver-registrar:v1.3.0,
trident-csi-zvrb2:      netapp/trident:20.07.0, quay.io/k8scsi/csi-node-driver-registrar:v1.3.0,
trident-operator-7f74ff5bb8-h88b6:      netapp/trident-operator:20.07.0,

The difference between Trident 20.04 and 20.07 is a new trident-autosupport sidecar container to periodically send Trident usage and support telemetry data to NetApp and an update to the csi-snapshotter.
What is also interesting to notice is that with newer releases of Kubernetes, new sidecars are added to CSI Trident:

• Kubernetes 1.16: Volume Expansion (CSI Resizer) was promoted to Beta status (https://kubernetes-csi.github.io/docs/volume-expansion.html)
• Kubernetes 1.17: Snapshot & Restore (CSI Snapshotter) was promoted to Beta status (https://kubernetes-csi.github.io/docs/snapshot-restore-feature.html)

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