Add help menu

Signed-off-by: dusanb <borovcanindusan1@gmail.com.com>

Author: dusanb

README.md

@@ -9,7 +9,7 @@ When running, please provide following format:
 
 | Flag | Description                                    | Default value    |
 | ---- | ---------------------------------------------- | ---------------- |
-| o    | observe   option - only valid with Get request | false            |
+| o    | observe   option - only valid with GET request | false            |
 | auth | auth option sent as URI Query                  | ""               |
 | h    | host                                           | "localhost"      |
 | p    | port                                           | "5683"           |

cmd/main.go

@@ -25,6 +25,25 @@ const (
 	delete = "DELETE"
 )
 
+const (
+	helpCmd = `Use "coap-cli --help" for help.`
+	helpMsg = `
+Usage: coap-cli <method> <URL> [options]
+mathod: get, put, post or delete
+-o      observe   option - only valid with GET request (default: false)
+-auth   auth option sent as URI Query                  (default: "")
+-h      host                                           (default: "localhost")
+-p      port                                           (default: "5683")
+-d      data to be sent in POST or PUT                 (default: "")
+-cf     content format                                 (default: 50 - JSON format))
+
+Examples:
+coap-cli get channels/0bb5ba61-a66e-4972-bab6-26f19962678f/messages/subtopic -auth 1e1017e6-dee7-45b4-8a13-00e6afeb66eb -o
+coap-cli post channels/0bb5ba61-a66e-4972-bab6-26f19962678f/messages/subtopic -auth 1e1017e6-dee7-45b4-8a13-00e6afeb66eb -d "hello world"
+coap-cli post channels/0bb5ba61-a66e-4972-bab6-26f19962678f/messages/subtopic -auth 1e1017e6-dee7-45b4-8a13-00e6afeb66eb -d "hello world" -h 0.0.0.0 -p 1234
+`
+)
+
 func parseCode(code string) (codes.Code, error) {
 	switch code {
 	case get:
@@ -47,26 +66,35 @@ func printMsg(m *pool.Message) {
 
 func main() {
 	if len(os.Args) < 2 {
-		log.Fatal("Message code must be GET, PUT, POST or DELETE")
+		log.Fatal(helpCmd)
+	}
+	help := strings.ToLower(os.Args[1])
+	if help == "-h" || help == "--help" {
+		log.Println(helpMsg)
+		os.Exit(0)
 	}
+
 	code, err := parseCode(strings.ToUpper(os.Args[1]))
 	if err != nil {
-		log.Fatal("error: ", err)
+		log.Fatalf("Can't read request code: %s\n%s", err, helpCmd)
 	}
+
 	if len(os.Args) < 3 {
-		log.Fatal("Please enter valid CoAP URL")
+		log.Fatalf("CoAP URL must not be empty.\n%s", helpCmd)
 	}
 	path := os.Args[2]
-	os.Args = os.Args[2:]
+	if strings.HasPrefix(path, "-") {
+		log.Fatalf("Please enter a valid CoAP URL.\n%s", helpCmd)
+	}
 
+	os.Args = os.Args[2:]
 	o := flag.Bool("o", false, "Observe")
 	h := flag.String("h", "localhost", "Host")
 	p := flag.String("p", "5683", "Port")
 	// Default type is JSON.
 	cf := flag.Int("q", 50, "Content format")
 	d := flag.String("d", "", "Message data")
 	a := flag.String("auth", "", "Auth token")
-
 	flag.Parse()
 
 	client, err := coap.New(*h + ":" + *p)