The kubectl command

1. The kubectl command

So, what exactly is the kubectl command and why is it important? kubectl is a utility used by administrators to control Kubernetes clusters. It's used to communicate with the Kube API server on the control plane. This is important to users, because it allows them to make requests to the cluster and kubectl determines which part of the control plane to communicate with. Within a selected Kubernetes cluster, kubectl transforms command-line entries into API calls and sends them to the Kube API server through HTTPS on the cluster's control plane server. However, to work properly, kubectl must be configured with the location and credentials of a Kubernetes cluster. And before kubectl can be used to configure a cluster, it must first be configured. kubectl stores its configuration in a file in the home directory in a hidden folder named .kube, and contains the list of clusters and the credentials that will be attached to each of those clusters. But where do the credentials come from? GKE provides them through the gcloud command. To view the configuration, either open the config file or use the kubectl command: "config view". Please note that the kubectl config shows the configuration of the kubectl command itself, whereas other kubectl commands show the configurations of cluster and workloads. For example, let's say an administrator wants to see a list of Pods in a cluster. After connecting kubectl to the cluster with proper credentials, the administrator can issue the kubectl "get pod" command. kubectl then converts this command into an API call, which it sends to the Kube API server through HTTPS on the cluster's control plane server. From there, the Kube API server processes the request by querying etcd. The Kube API server then returns the results to kubectl through HTTPS. Finally, kubectl interprets the API response and displays the results to the administrator at the command prompt. To connect kubectl to a GKE cluster, first retrieve the credentials for the specified cluster. This can be done with the "get-credentials" gcloud command in any other environment where the gcloud command-line tool and kubectl are installed. Note that both are installed by default in Cloud Shell. By default, the gcloud "get-credentials" command writes configuration information into a config file in the .kube directory in the $HOME directory. If this command is rerun for a different cluster, it'll update the config file with the credentials for the new cluster. This configuration process only needs to be performed once per cluster in Cloud Shell, because the .kube directory and its contents stay in the $HOME directory. The gcloud command is how authorized users interact with Google Cloud from the command line. If authorized, the gcloud "get-credentials" command provides the credentials needed to connect with a GKE cluster. Although kubectl is a tool for administering the internal state of an existing cluster, it can't create new clusters or change the shape of existing clusters. That's done through the GKE control plane, which the gcloud command and the Google Cloud console interfaces to. After the config file in the .kube folder is configured, the kubectl command automatically references this file and connects to the default cluster without prompting for credentials. Now let's explore how to use the kubectl command. kubectl's syntax is composed of four parts: the command, the type, the name, and optional flags. The command specifies the action that you want to perform, such as get, describe, logs, or exec. Some commands show information, whereas others change the cluster's configuration. The TYPE defines the Kubernetes object that the "command" acts upon, like Pods, deployments, nodes, or other objects, including the cluster itself. The TYPE used in combination with "command" tells kubectl what you want to do and the type of object you want to perform that action on. The NAME specifies the object defined in TYPE. The name field isn't always needed, especially when using commands that list or show information. For example, if you run the command "kubectl get pods" without specifying a name, the command returns the list of all Pods. To filter this list, specify a Pod's name, such as "kubectl get pod my-test-app" and kubectl will return only information on the Pod named 'my-test-app'. Some commands let you append flags to the end, which you can think of as a way to make a special request. For example, to view the state of a Pod, use the command "kubectl get pod my-test-app -o=yaml". Also, it's worth mentioning that telling kubectl to produce a YAML output can be helpful for other tasks related to Kubernetes objects, for example, recreating an object in another cluster. Flags can also be used to display additional information. For example, run the command "kubectl get pods -o=wide" to display the list of Pods in "wide" format, which reveals Pods in the list. Wide format also displays which node each Pod is running on. The kubectl command has many uses, from creating Kubernetes objects, to viewing them, deleting them, and viewing or exporting configuration files. Just remember to configure kubectl first or to use the --kubeconfig or --context parameters, so that the commands you type are performed on the cluster you intended.

2. Let's practice!

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