Create a kubectl alpha subcommand

Alpha commands can be added under `kubectl alpha` and are always
accessible (regardless of feature gates). If no alpha commands have been
defined then `alpha` is not displayed in `help`.

pkg/kubectl/cmd/cmd.go

@@ -354,6 +354,13 @@ func NewKubectlCommand(f cmdutil.Factory, in io.Reader, out, err io.Writer) *cob
 		"options",
 		Deprecated("kubectl", "delete", cmds, NewCmdStop(f, out)),
 	}
+
+	// Hide the "alpha" subcommand if there are no alpha commands in this build.
+	alpha := NewCmdAlpha(f, in, out, err)
+	if !alpha.HasSubCommands() {
+		filters = append(filters, alpha.Name())
+	}
+
 	templates.ActsAsRootCommand(cmds, filters, groups...)
 
 	for name, completion := range bash_completion_flags {
@@ -368,6 +375,7 @@ func NewKubectlCommand(f cmdutil.Factory, in io.Reader, out, err io.Writer) *cob
 		}
 	}
 
+	cmds.AddCommand(alpha)
 	cmds.AddCommand(cmdconfig.NewCmdConfig(clientcmd.NewDefaultPathOptions(), out, err))
 	cmds.AddCommand(NewCmdPlugin(f, in, out, err))
 	cmds.AddCommand(NewCmdVersion(f, out))