command improvements

cmd/slinguist/main.go

@@ -13,10 +13,19 @@ import (
 
 func main() {
 	flag.Parse()
-	root := flag.Arg(0)
+	root, err := filepath.Abs(flag.Arg(0))
+	ifError(err)
+
+	if root == "" {
+		usage()
+	}
 
 	o := make(map[string][]string, 0)
-	filepath.Walk(root, func(path string, f os.FileInfo, err error) error {
+	err = filepath.Walk(root, func(path string, f os.FileInfo, err error) error {
+		if err != nil {
+			return err
+		}
+
 		if slinguist.IsVendor(f.Name()) || slinguist.IsDotFile(f.Name()) {
 			if f.IsDir() {
 				return filepath.SkipDir
@@ -36,10 +45,30 @@ func main() {
 
 		}
 
-		o[l] = append(o[l], path)
+		r, _ := filepath.Rel(root, path)
+		o[l] = append(o[l], r)
 		return nil
 	})
 
+	ifError(err)
+
 	js, _ := json.MarshalIndent(o, "", "  ")
 	fmt.Printf("%s\n", js)
 }
+
+func usage() {
+	fmt.Fprintf(
+		os.Stderr, "simple-linguist, A simple (and faster) implementation of linguist \nusage: %s <path>\n",
+		os.Args[0],
+	)
+
+	flag.PrintDefaults()
+	os.Exit(2)
+}
+
+func ifError(err error) {
+	if err != nil {
+		fmt.Fprintf(os.Stderr, err.Error())
+		os.Exit(2)
+	}
+}