Add basic usage of flag package

Author: tmpbook

flag/main.go

@@ -0,0 +1,33 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+)
+
+var (
+	// Return a point which can read by *name after flag.Parse() executed
+	name = flag.String("name", "unknow", "your name")
+)
+
+func main() {
+	flag.Parse()
+
+	// Create a visitor function, execute with a *flag.Flag callback param for each flag
+	visitor := func(f *flag.Flag) {
+		fmt.Println("option =", f.Name, " value =", f.Value)
+	}
+	flag.VisitAll(visitor)
+
+	fmt.Println(*name)
+}
+
+// cmd   : go run main.go -name tmpbook
+// output:
+// 	   option = name  value = tmpbook
+// 	   tmpbook
+
+// cmd   : go run main.go
+// output:
+// 	   option = name  value = unknow
+// 	   unknow