diff --git a/src/study/day6/practice4.2.go b/src/study/day6/practice4.2.go
new file mode 100644
index 0000000..78fa2c8
--- /dev/null
+++ b/src/study/day6/practice4.2.go
@@ -0,0 +1,42 @@
+package main
+
+import (
+	"crypto/sha256"
+	"crypto/sha512"
+	"flag"
+	"fmt"
+	"os"
+)
+
+// 练习 4.2： 编写一个程序，默认情况下打印标准输入的SHA256编码，
+// 并支持通过命令行flag定制，输出SHA384或SHA512哈希算法。
+func main() {
+	var shaflag = flag.Int("sha", 256, "sha选择")
+	var str = flag.String("s", "default", "需要被加密的字符串")
+	flag.Parse()
+	fmt.Println(*shaflag, *str)
+
+	sha := func() []byte {
+		if *shaflag == 256 {
+			s := sha256.Sum256([]byte(*str))
+			res := make([]byte, len(s))
+			copy(res, s[:])
+			return res
+		} else if *shaflag == 384 {
+			s := sha512.Sum384([]byte(*str))
+			res := make([]byte, len(s))
+			copy(res, s[:])
+			return res
+		} else if *shaflag == 512 {
+			s := sha512.Sum512([]byte(*str))
+			res := make([]byte, len(s))
+			copy(res, s[:])
+			return res
+		} else {
+			fmt.Println("参数错误，请输入256，384，512")
+			os.Exit(0)
+		}
+		return []byte{0}
+	}
+	fmt.Printf("%T \n%[1]x \n", sha())
+}