kubens: Start implementing stubs

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

cmd/kubens/help.go

@@ -0,0 +1,44 @@
+package main
+
+import (
+	"fmt"
+	"io"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"github.com/pkg/errors"
+)
+
+// HelpOp describes printing help.
+type HelpOp struct{}
+
+func (_ HelpOp) Run(stdout, _ io.Writer) error {
+	return printUsage(stdout)
+}
+
+func printUsage(out io.Writer) error {
+	help := `USAGE:
+  %PROG%                    : list the namespaces in the current context
+  %PROG% <NAME>             : change the active namespace of current context
+  %PROG% -                  : switch to the previous namespace in this context
+  %PROG% -c, --current      : show the current namespace
+  %PROG% -h,--help          : show this message
+`
+	// TODO this replace logic is duplicated between this and kubectx
+	help = strings.ReplaceAll(help, "%PROG%", selfName())
+
+	_, err := fmt.Fprintf(out, "%s\n", help)
+	return errors.Wrap(err, "write error")
+}
+
+// selfName guesses how the user invoked the program.
+func selfName() string {
+	// TODO this method is duplicated between this and kubectx
+	me := filepath.Base(os.Args[0])
+	pluginPrefix := "kubectl-"
+	if strings.HasPrefix(me, pluginPrefix) {
+		return "kubectl " + strings.TrimPrefix(me, pluginPrefix)
+	}
+	return "kubectx"
+}