diff --git a/go/study/ch03_file_io/go.mod b/go/study/ch03_file_io/go.mod
new file mode 100644
index 00000000..816ff56c
--- /dev/null
+++ b/go/study/ch03_file_io/go.mod
@@ -0,0 +1,3 @@
+module github.com/tecyokomichi/WebAppLearning/go/study/file_io
+
+go 1.19
diff --git a/go/study/ch03_file_io/main.go b/go/study/ch03_file_io/main.go
new file mode 100644
index 00000000..9369a699
--- /dev/null
+++ b/go/study/ch03_file_io/main.go
@@ -0,0 +1,78 @@
+package main
+
+import (
+	"fmt"
+	"os"
+	"strings"
+)
+
+func main() {
+	
+	switch os.Args[1] {
+	case "cat":
+		if len(os.Args) != 3 {
+			showHelp()
+			os.Exit(1)
+		}
+		f, err := os.ReadFile(os.Args[2])
+		if err != nil {
+			fmt.Fprintln(os.Stderr, err)
+			os.Exit(1)
+		}
+		fmt.Println(string(f))
+	case "write":
+		if len (os.Args) != 4 {
+			showHelp()
+			os.Exit(1)
+		}
+		if _, err := os.Stat(os.Args[2]); err != nil {
+			if !os.IsNotExist(err) {
+				fmt.Fprintln(os.Stderr, err)
+				os.Exit(1)
+			}
+		} else {
+			fmt.Printf("%s already exists. Overwrite? (y/n): ", os.Args[2])
+			var res string
+			fmt.Scanln(&res)
+			if !strings.HasPrefix(strings.ToLower(res), "y") {
+				fmt.Println("Quit writing")
+				os.Exit(1)
+			}
+		}
+		f, err := os.Create(os.Args[2])
+		if err != nil {
+			fmt.Fprintln(os.Stderr, err)
+			os.Exit(1)
+		}
+		defer f.Close()
+		_, ee := fmt.Fprintln(f, os.Args[3])
+		if ee != nil{
+			fmt.Fprintln(os.Stderr, ee)
+			os.Exit(1)
+		}
+	case "append":
+		if len (os.Args) != 4 {
+			showHelp()
+			os.Exit(1)
+		}
+		f, err := os.OpenFile(os.Args[2], os.O_WRONLY|os.O_APPEND|os.O_CREATE, 0664)
+		if err != nil {
+			fmt.Fprintln(os.Stderr, err)
+			os.Exit(1)
+		}
+		defer f.Close()
+		_, e := fmt.Fprintln(f, os.Args[3])
+		if e != nil{
+			fmt.Fprintln(os.Stderr, e)
+			os.Exit(1)
+		}
+	default:
+		showHelp()
+		os.Exit(1)
+	}
+}
+
+func showHelp() {
+	fmt.Printf("USAGE:\n")
+	fmt.Printf("%s cat FILE\n", os.Args[0])
+}