Added check for kubectl to each command (in root.go) This prompts user

with kubectl install one-liner for latest kubectl version.  Also added
config for enable/disable.  Still need to add tests.

cmd/minikube/cmd/config/config.go

@@ -108,6 +108,10 @@ var settings = []Setting{
 		name: config.WantReportErrorPrompt,
 		set:  SetBool,
 	},
+	{
+		name: config.WantKubectlDownloadMsg,
+		set:  SetBool,
+	},
 	{
 		name:        "dashboard",
 		set:         SetBool,

cmd/minikube/cmd/root.go

@@ -28,6 +28,7 @@ import (
 	"github.com/spf13/pflag"
 	"github.com/spf13/viper"
 	configCmd "k8s.io/minikube/cmd/minikube/cmd/config"
+	"k8s.io/minikube/cmd/util"
 	"k8s.io/minikube/pkg/minikube/config"
 	"k8s.io/minikube/pkg/minikube/constants"
 	"k8s.io/minikube/pkg/minikube/notify"
@@ -51,6 +52,7 @@ const (
 
 var (
 	enableUpdateNotification = true
+	enableKubectlDownloadMsg = true
 )
 
 var viperWhiteList = []string{
@@ -83,6 +85,9 @@ var RootCmd = &cobra.Command{
 		if enableUpdateNotification {
 			notify.MaybePrintUpdateTextFromGithub(os.Stdout)
 		}
+		if enableKubectlDownloadMsg {
+			util.MaybePrintKubectlDownloadMsg()
+		}
 	},
 }
 
@@ -147,5 +152,6 @@ func setupViper() {
 	viper.SetDefault(config.ReminderWaitPeriodInHours, 24)
 	viper.SetDefault(config.WantReportError, false)
 	viper.SetDefault(config.WantReportErrorPrompt, true)
+	viper.SetDefault(config.WantKubectlDownloadMsg, true)
 	setFlagsUsingViper()
 }

cmd/util/util.go

@@ -24,6 +24,8 @@ import (
 	"io"
 	"net/http"
 	"os"
+	"os/exec"
+	"runtime"
 	"strings"
 	"time"
 
@@ -165,3 +167,33 @@ func PromptUserForAccept(r io.Reader) bool {
 		return false
 	}
 }
+
+func MaybePrintKubectlDownloadMsg() {
+	if !viper.GetBool(config.WantKubectlDownloadMsg) {
+		return
+	}
+	verb := "run"
+	installInstructions := "curl -Lo kubectl https://storage.googleapis.com/kubernetes-release/release/%s/bin/%s/%s/kubectl && chmod +x kubectl && sudo mv kubectl /usr/local/bin/"
+	if runtime.GOOS == "windows" {
+		verb = "do"
+		installInstructions = `download kubectl from:
+https://storage.googleapis.com/kubernetes-release/release/%s/bin/%s/%s/kubectl
+Add kubectl to your system PATH`
+	}
+
+	_, err := exec.LookPath("kubectl")
+	if err != nil {
+		fmt.Fprintf(os.Stderr,
+			`========================================
+kubectl could not be found on your path.  kubectl is a requirement for using minikube
+To install kubectl, please %s the following:
+
+%s
+
+To disable this message, run the following:
+
+minikube config set WantKubectlDownloadMsg false
+========================================\n`,
+			verb, installInstructions, constants.DefaultKubernetesVersion, runtime.GOOS, runtime.GOARCH)
+	}
+}

docs/minikube_config.md

@@ -21,6 +21,7 @@ Configurable fields:
  * ReminderWaitPeriodInHours
  * WantReportError
  * WantReportErrorPrompt
+ * WantKubectlDownloadMsg
  * dashboard
  * addon-manager
  * kube-dns

pkg/minikube/config/config.go

@@ -31,6 +31,7 @@ const (
 	ReminderWaitPeriodInHours = "ReminderWaitPeriodInHours"
 	WantReportError           = "WantReportError"
 	WantReportErrorPrompt     = "WantReportErrorPrompt"
+	WantKubectlDownloadMsg    = "WantKubectlDownloadMsg"
 )
 
 type configFile interface {