kubens: Start implementing stubs

Signed-off-by: Ahmet Alp Balkan <ahmetb@google.com>

cmd/kubens/flags.go

@@ -0,0 +1,40 @@
+package main
+
+import (
+	"fmt"
+	"io"
+	"strings"
+)
+
+// UnsupportedOp indicates an unsupported flag.
+type UnsupportedOp struct{ Err error }
+
+func (op UnsupportedOp) Run(_, _ io.Writer) error {
+	return op.Err
+}
+
+// parseArgs looks at flags (excl. executable name, i.e. argv[0])
+// and decides which operation should be taken.
+func parseArgs(argv []string) Op {
+	if len(argv) == 0 {
+		//if env.IsInteractiveMode(os.Stdout) {
+		//	return InteractiveSwitchOp{SelfCmd: os.Args[0]}
+		//}
+		return ListOp{}
+	}
+
+	if len(argv) == 1 {
+		v := argv[0]
+		if v == "--help" || v == "-h" {
+			return HelpOp{}
+		}
+		if v == "--current" || v == "-c" {
+			return CurrentOp{}
+		}
+		if strings.HasPrefix(v, "-") && v != "-" {
+			return UnsupportedOp{Err: fmt.Errorf("unsupported option '%s'", v)}
+		}
+		return SwitchOp{Target: argv[0]}
+	}
+	return UnsupportedOp{Err: fmt.Errorf("too many arguments")}
+}