update book code

eBook/examples/chapter_12/echo.go

@@ -0,0 +1,29 @@
+package main
+
+import (
+	"os"
+	"flag" // command line option parser
+)
+
+var NewLine = flag.Bool("n", false, "print newline") // echo -n flag, of type *bool
+
+const (
+	Space   = " "
+	Newline = "\n"
+)
+
+func main() {
+	flag.PrintDefaults()
+	flag.Parse() // Scans the arg list and sets up flags
+	var s string = ""
+	for i := 0; i < flag.NArg(); i++ {
+		if i > 0 {
+			s += " "
+			if *NewLine {  // -n is parsed, flag becomes true
+				s += Newline
+			}
+		}
+		s += flag.Arg(i)
+	}
+	os.Stdout.WriteString(s)
+}