apiVersion: v1 
kind: Pod 
metadata: 
  name: nginx-pod 
spec:    
  containers:    
  - name: nginx     
    image: nginx:1.8 

kubectl create -f nginx_pod.yaml 
pod/nginx-pod created

kubectl get pods  
NAME                     READY   STATUS              RESTARTS   AGE 
nginx-pod                1/1     Running             0          4m22s 

func main() {
 	// 如果不調用rand.Seed，每次重新運行這個main函數，rand下的函數返回值始終一致
	// Seed即隨機的種子，每次用時間戳作為種子，就能保證隨機性
	rand.Seed(time.Now().UnixNano())
        // 創建了kubectl命令的默認參數
	command := cmd.NewDefaultKubectlCommand()
	// TODO: once we switch everything over to Cobra commands, we can go back to calling
	// cliflag.InitFlags() (by removing its pflag.Parse() call). For now, we have to set the
	// normalize func and add the go flag set by hand.
	pflag.CommandLine.SetNormalizeFunc(cliflag.WordSepNormalizeFunc)
	pflag.CommandLine.AddGoFlagSet(goflag.CommandLine)
	// cliflag.InitFlags()
        // 日志的初始化與退出
	logs.InitLogs()
	defer logs.FlushLogs()
        // 運行command
	if err := command.Execute(); err != nil {
		os.Exit(1)
	}
}

// k8s的命令行工具采用了 cobra 庫，具有命令提示等強大功能，比go語言自帶的flag強大很多，可參考 github.com/spf13/cobra
func NewDefaultKubectlCommand() *cobra.Command {
	return NewDefaultKubectlCommandWithArgs(NewDefaultPluginHandler(plugin.ValidPluginFilenamePrefixes), os.Args, os.Stdin, os.Stdout, os.Stderr)
}
func NewDefaultKubectlCommandWithArgs(pluginHandler PluginHandler, args []string, in io.Reader, out, errout io.Writer) *cobra.Command {
  // 初始化NewKubectlCommand，采用標準輸入、輸出、錯誤輸出
	cmd := NewKubectlCommand(in, out, errout)
	if pluginHandler == nil {
		return cmd
	}
	if len(args) > 1 {
    // 這里為傳入的參數，即 create -f nginx_pod.yaml 部分
		cmdPathPieces := args[1:]
		// 調用cobra的Find去匹配args
		if _, _, err := cmd.Find(cmdPathPieces); err != nil {
			if err := HandlePluginCommand(pluginHandler, cmdPathPieces); err != nil {
				fmt.Fprintf(errout, "%v\n", err)
				os.Exit(1)
			}
		}
	}
	return cmd
}

func NewKubectlCommand(in io.Reader, out, err io.Writer) *cobra.Command {
	warningHandler := rest.NewWarningWriter(err, rest.WarningWriterOptions{Deduplicate: true, Color: term.AllowsColorOutput(err)})
	warningsAsErrors := false
	// 創建主命令 告訴你kubectl該怎么用
	cmds := &cobra.Command{
		Use:   "kubectl",
		Short: i18n.T("kubectl controls the Kubernetes cluster manager"),
		Long: templates.LongDesc(`
      kubectl controls the Kubernetes cluster manager.
      Find more information at:
            https://kubernetes.io/docs/reference/kubectl/overview/`),
		Run: runHelp,
		// 初始化后，在運行指令前的鉤子
		PersistentPreRunE: func(*cobra.Command, []string) error {
			rest.SetDefaultWarningHandler(warningHandler)
      // 這里是做pprof性能分析，跳轉到對應代碼可以看到，我們可以用參數 --profile xxx 來采集性能指標，默認保存在當前目錄下的profile.pprof中
			return initProfiling()
		},
    // 運行指令后的鉤子
		PersistentPostRunE: func(*cobra.Command, []string) error {
      // 保存pprof性能分析指標
			if err := flushProfiling(); err != nil {
				return err
			}
      // 打印warning條數
			if warningsAsErrors {
				count := warningHandler.WarningCount()
				switch count {
				case 0:
					// no warnings
				case 1:
					return fmt.Errorf("%d warning received", count)
				default:
					return fmt.Errorf("%d warnings received", count)
				}
			}
			return nil
		},
    // bash自動補齊功能，可通過 kubectl completion bash 命令查看
    // 具體安裝可參考 https://kubernetes.io/docs/tasks/tools/install-kubectl/#enabling-shell-autocompletion
		BashCompletionFunction: bashCompletionFunc,
	}
  // 實例化Factory接口，工廠模式
	f := cmdutil.NewFactory(matchVersionKubeConfigFlags)
	// 省略實例化的過程代碼
  // kubectl定義了7類命令，結合Message和各個子命令的package名來看
	groups := templates.CommandGroups{
		{
      // 1. 初級命令，包括 create/expose/run/set   
			Message: "Basic Commands (Beginner):",
			Commands: []*cobra.Command{
				create.NewCmdCreate(f, ioStreams),
				expose.NewCmdExposeService(f, ioStreams),
				run.NewCmdRun(f, ioStreams),
				set.NewCmdSet(f, ioStreams),
			},
		},
		{
      // 2. 中級命令，包括explain/get/edit/delete
			Message: "Basic Commands (Intermediate):",
			Commands: []*cobra.Command{
				explain.NewCmdExplain("kubectl", f, ioStreams),
				get.NewCmdGet("kubectl", f, ioStreams),
				edit.NewCmdEdit(f, ioStreams),
				delete.NewCmdDelete(f, ioStreams),
			},
		},
		{
      // 3. 部署命令，包括 rollout/scale/autoscale
			Message: "Deploy Commands:",
			Commands: []*cobra.Command{
				rollout.NewCmdRollout(f, ioStreams),
				scale.NewCmdScale(f, ioStreams),
				autoscale.NewCmdAutoscale(f, ioStreams),
			},
		},
		{
      // 4. 集群管理命令，包括 cerfificate/cluster-info/top/cordon/drain/taint
			Message: "Cluster Management Commands:",
			Commands: []*cobra.Command{
				certificates.NewCmdCertificate(f, ioStreams),
				clusterinfo.NewCmdClusterInfo(f, ioStreams),
				top.NewCmdTop(f, ioStreams),
				drain.NewCmdCordon(f, ioStreams),
				drain.NewCmdUncordon(f, ioStreams),
				drain.NewCmdDrain(f, ioStreams),
				taint.NewCmdTaint(f, ioStreams),
			},
		},
		{
      // 5. 故障排查和調試，包括 describe/logs/attach/exec/port-forward/proxy/cp/auth
			Message: "Troubleshooting and Debugging Commands:",
			Commands: []*cobra.Command{
				describe.NewCmdDescribe("kubectl", f, ioStreams),
				logs.NewCmdLogs(f, ioStreams),
				attach.NewCmdAttach(f, ioStreams),
				cmdexec.NewCmdExec(f, ioStreams),
				portforward.NewCmdPortForward(f, ioStreams),
				proxy.NewCmdProxy(f, ioStreams),
				cp.NewCmdCp(f, ioStreams),
				auth.NewCmdAuth(f, ioStreams),
			},
		},
		{
      // 6. 高級命令，包括diff/apply/patch/replace/wait/convert/kustomize
			Message: "Advanced Commands:",
			Commands: []*cobra.Command{
				diff.NewCmdDiff(f, ioStreams),
				apply.NewCmdApply("kubectl", f, ioStreams),
				patch.NewCmdPatch(f, ioStreams),
				replace.NewCmdReplace(f, ioStreams),
				wait.NewCmdWait(f, ioStreams),
				convert.NewCmdConvert(f, ioStreams),
				kustomize.NewCmdKustomize(ioStreams),
			},
		},
		{
      // 7. 設置命令，包括label，annotate，completion
			Message: "Settings Commands:",
			Commands: []*cobra.Command{
				label.NewCmdLabel(f, ioStreams),
				annotate.NewCmdAnnotate("kubectl", f, ioStreams),
				completion.NewCmdCompletion(ioStreams.Out, ""),
			},
		},
	}
	groups.Add(cmds)
	filters := []string{"options"}
	// alpha相關的子命令
	alpha := cmdpkg.NewCmdAlpha(f, ioStreams)
	if !alpha.HasSubCommands() {
		filters = append(filters, alpha.Name())
	}
	templates.ActsAsRootCommand(cmds, filters, groups...)
  // 代碼補全相關
	for name, completion := range bashCompletionFlags {
		if cmds.Flag(name) != nil {
			if cmds.Flag(name).Annotations == nil {
				cmds.Flag(name).Annotations = map[string][]string{}
			}
			cmds.Flag(name).Annotations[cobra.BashCompCustom] = append(
				cmds.Flag(name).Annotations[cobra.BashCompCustom],
				completion,
			)
		}
	}
  // 添加其余子命令，包括 alpha/config/plugin/version/api-versions/api-resources/options
	cmds.AddCommand(alpha)
	cmds.AddCommand(cmdconfig.NewCmdConfig(f, clientcmd.NewDefaultPathOptions(), ioStreams))
	cmds.AddCommand(plugin.NewCmdPlugin(f, ioStreams))
	cmds.AddCommand(version.NewCmdVersion(f, ioStreams))
	cmds.AddCommand(apiresources.NewCmdAPIVersions(f, ioStreams))
	cmds.AddCommand(apiresources.NewCmdAPIResources(f, ioStreams))
	cmds.AddCommand(options.NewCmdOptions(ioStreams.Out))
	return cmds
}

func NewCmdCreate(f cmdutil.Factory, ioStreams genericclioptions.IOStreams) *cobra.Command {
  // create子命令的相關選項
	o := NewCreateOptions(ioStreams)
  // create子命令的相關說明
	cmd := &cobra.Command{
		Use:                   "create -f FILENAME",
		DisableFlagsInUseLine: true,
		Short:                 i18n.T("Create a resource from a file or from stdin."),
		Long:                  createLong,
		Example:               createExample,
    // 驗證參數并運行
		Run: func(cmd *cobra.Command, args []string) {
			if cmdutil.IsFilenameSliceEmpty(o.FilenameOptions.Filenames, o.FilenameOptions.Kustomize) {
				ioStreams.ErrOut.Write([]byte("Error: must specify one of -f and -k\n\n"))
				defaultRunFunc := cmdutil.DefaultSubCommandRun(ioStreams.ErrOut)
				defaultRunFunc(cmd, args)
				return
			}
			cmdutil.CheckErr(o.Complete(f, cmd))
			cmdutil.CheckErr(o.ValidateArgs(cmd, args))
      // 核心的運行代碼邏輯是在這里的RunCreate
			cmdutil.CheckErr(o.RunCreate(f, cmd))
		},
	}
	o.RecordFlags.AddFlags(cmd)
	usage := "to use to create the resource"
  // 加入文件名選項的flag -f，保存到o.FilenameOptions.Filenames中，對應上面
	cmdutil.AddFilenameOptionFlags(cmd, &o.FilenameOptions, usage)
	cmdutil.AddValidateFlags(cmd)
	cmd.Flags().BoolVar(&o.EditBeforeCreate, "edit", o.EditBeforeCreate, "Edit the API resource before creating")
	cmd.Flags().Bool("windows-line-endings", runtime.GOOS == "windows",
		"Only relevant if --edit=true. Defaults to the line ending native to your platform.")
	cmdutil.AddApplyAnnotationFlags(cmd)
	cmdutil.AddDryRunFlag(cmd)
	cmd.Flags().StringVarP(&o.Selector, "selector", "l", o.Selector, "Selector (label query) to filter on, supports '=', '==', and '!='.(e.g. -l key1=value1,key2=value2)")
	cmd.Flags().StringVar(&o.Raw, "raw", o.Raw, "Raw URI to POST to the server.  Uses the transport specified by the kubeconfig file.")
	cmdutil.AddFieldManagerFlagVar(cmd, &o.fieldManager, "kubectl-create")
	o.PrintFlags.AddFlags(cmd)
	// create的子命令，指定create對象
	cmd.AddCommand(NewCmdCreateNamespace(f, ioStreams))
	cmd.AddCommand(NewCmdCreateQuota(f, ioStreams))
	cmd.AddCommand(NewCmdCreateSecret(f, ioStreams))
	cmd.AddCommand(NewCmdCreateConfigMap(f, ioStreams))
	cmd.AddCommand(NewCmdCreateServiceAccount(f, ioStreams))
	cmd.AddCommand(NewCmdCreateService(f, ioStreams))
	cmd.AddCommand(NewCmdCreateDeployment(f, ioStreams))
	cmd.AddCommand(NewCmdCreateClusterRole(f, ioStreams))
	cmd.AddCommand(NewCmdCreateClusterRoleBinding(f, ioStreams))
	cmd.AddCommand(NewCmdCreateRole(f, ioStreams))
	cmd.AddCommand(NewCmdCreateRoleBinding(f, ioStreams))
	cmd.AddCommand(NewCmdCreatePodDisruptionBudget(f, ioStreams))
	cmd.AddCommand(NewCmdCreatePriorityClass(f, ioStreams))
	cmd.AddCommand(NewCmdCreateJob(f, ioStreams))
	cmd.AddCommand(NewCmdCreateCronJob(f, ioStreams))
	return cmd
}

func (o *CreateOptions) RunCreate(f cmdutil.Factory, cmd *cobra.Command) error {
	// f為傳入的Factory，主要是封裝了與kube-apiserver交互客戶端
	schema, err := f.Validator(cmdutil.GetFlagBool(cmd, "validate"))
	if err != nil {
		return err
	}
	cmdNamespace, enforceNamespace, err := f.ToRawKubeConfigLoader().Namespace()
	if err != nil {
		return err
	}
  // 實例化Builder，這塊的邏輯比較復雜，我們先關注文件部分 這些大部分是給builder設置參數，在Do的時候執行邏輯，返回我們想要的Result
	r := f.NewBuilder().
		Unstructured().
		Schema(schema).
		ContinueOnError().
		NamespaceParam(cmdNamespace).DefaultNamespace().
  	// 讀取文件信息，發現除了支持簡單的本地文件，也支持標準輸入和http/https協議訪問的文件，保存為Visitor 
		FilenameParam(enforceNamespace, &o.FilenameOptions).
		LabelSelectorParam(o.Selector).
		Flatten().
		Do()
	err = r.Err()
	if err != nil {
		return err
	}
	count := 0
  // 調用visit函數，創建資源
	err = r.Visit(func(info *resource.Info, err error) error {
		// 我們看到的pod創建成功就是在這里打印的 打印結果 xxxx created 
		return o.PrintObj(info.Object)
	})
	return nil
}