fix: test if something being pipe to stdin

cmd/mark/main.go

@@ -45,11 +45,11 @@ func main() {
 		defer file.Close()
 		reader = bufio.NewReader(file)
 	} else {
-		if stat, err := os.Stdin.Stat(); err == nil && stat.Size() > 0 {
-			reader = bufio.NewReader(os.Stdin)
-		} else {
+		stat, err := os.Stdin.Stat()
+		if err != nil || (stat.Mode()&os.ModeCharDevice) != 0 {
 			usageAndExit("")
 		}
+		reader = bufio.NewReader(os.Stdin)
 	}
 	// collect data
 	var data string
@@ -65,8 +65,8 @@ func main() {
 	}
 	// write
 	var (
-		err error
-		file *os.File = os.Stdout
+		err  error
+		file = os.Stdout
 	)
 	if *output != "" {
 		if file, err = os.Create(*output); err != nil {
@@ -91,4 +91,4 @@ func usageAndExit(msg string) {
 	flag.Usage()
 	fmt.Fprintf(os.Stderr, "\n")
 	os.Exit(1)
-}
+}