Originally posted by Gianluca Arbezzano on Gianarb.it
This is not at all a new topic, no hacking involved, but it is something everybody needs to know where we design kubectl plugin.
I was recently working at one and I had to make the user experience as friendly as possible compared with the kubectl, because that’s what a good developer does! Tricks other developers to make their life comfortable, if you are used to do:
$ kubectl get pod -n your-namespace -L app=http
To get pods from a particular namespace your-namemespace filtered by label app=http and your plugin does something similar or it will benefit from an interaction that remembers the classic get you should re-use those flags.
Example: design a kubectl-plugin capable of running a pprof profile against a set of containers.
My expectation will be to do something like:
$ kubectl pprof -n your-namespace -n pod-name-go-app
The Kubernetes community writes a lot of their code in Go, it means that there are a lot of libraries that you can re-use.
kubernetes/cli-runtime is a library that provides utilities to create kubectl plugins. One of their packages is called genericclioptions and as you can get from its name the goal is obvious.
// import "github.com/spf13/cobra"
// import "github.com/spf13/pflag"
// import "k8s.io/cli-runtime/pkg/genericclioptions"
// Create the set of flags for your kubect-plugin
flags := pflag.NewFlagSet("kubectl-plugin", pflag.ExitOnError)
pflag.CommandLine = flags
// Configure the genericclioptions
streams := genericclioptions.IOStreams{
In: os.Stdin,
Out: os.Stdout,
ErrOut: os.Stderr,
}
// This set of flags is the one used for the kubectl configuration such as:
// cache-dir, cluster-name, namespace, kube-config, insecure, timeout, impersonate,
// ca-file and so on
kubeConfigFlags := genericclioptions.NewConfigFlags(false)
// It is a set of flags related to a specific resource such as: label selector
(-L), --all-namespaces, --schema and so on.
kubeResouceBuilderFlags := genericclioptions.NewResourceBuilderFlags()
var rootCmd = &cobra.Command{
Use: "kubectl-plugin",
Short: "My root command",
Run: func(cmd *cobra.Command, args []string) {
cmd.SetOutput(streams.ErrOut)
}
}
// You can join all this flags to your root command
flags.AddFlagSet(rootCmd.PersistentFlags())
kubeConfigFlags.AddFlags(flags)
kubeResouceBuilderFlags.AddFlags(flags)
This is the output:
$ kubectl-plugin --help
My root command
Usage:
kubectl-plugin [flags]
Flags:
--as string Username to impersonate for the operation
--as-group stringArray Group to impersonate for the operation, this flag can be repeated to specify multiple groups.
--cache-dir string Default HTTP cache directory (default "/home/gianarb/.kube/http-cache")
--certificate-authority string Path to a cert file for the certificate authority
--client-certificate string Path to a client certificate file for TLS
--client-key string Path to a client key file for TLS
--cluster string The name of the kubeconfig cluster to use
--context string The name of the kubeconfig context to use
-f, --filename strings identifying the resource.
-h, --help help for kubectl-pprof
--insecure-skip-tls-verify If true, the server's certificate will not be checked for validity. This will make your HTTPS connections insecure
--kubeconfig string Path to the kubeconfig file to use for CLI requests.
-n, --namespace string If present, the namespace scope for this CLI request
-R, --recursive Process the directory used in -f, --filename recursively. Useful when you want to manage related manifests organized within the same directory. (default true)
--request-timeout string The length of time to wait before giving up on a single server request. Non-zero values should contain a corresponding time unit (e.g. 1s, 2m, 3h). A value of zero means don't timeout requests. (default "0")
-l, --selector string Selector (label query) to filter on, supports '=', '==', and '!='.(e.g. -l key1=value1,key2=value2)
-s, --server string The address and port of the Kubernetes API server
--token string Bearer token for authentication to the API server
--user string The name of the kubeconfig user to use