Implement #RA6

app/main.go

 	CommandTypeBuiltinEcho
 	CommandTypeBuiltinType
 	CommandTypeBuiltinPwd
+	CommandTypeBuiltinCd
 )
 
 func getCommand(ctx *Context) (Command, error) {
 	if tokens[0] == "pwd" {
 		return makeCommandBuiltin(CommandTypeBuiltinPwd, tokens), nil
 	}
+	if tokens[0] == "cd" {
+		return makeCommandBuiltin(CommandTypeBuiltinCd, tokens), nil
+	}
 	if tokens[0] == "echo" {
 		return makeCommandBuiltin(CommandTypeBuiltinEcho, tokens), nil
 	}
 	return makeCommandExternal(tokens, exec_path), nil
 }
 
+func bltnChdir(ctx Context, path string) ExitStatus {
+	err := os.Chdir(path)
+	if err != nil {
+		fmt.Fprintf(ctx.stderr, "cd: %s: No such file or directory\n", path)
+		return 4
+	}
+	return 0
+}
+
 func handleCommand(ctx Context, command Command) ExitStatus {
 	// fmt.Printf("handleCommand():command=%#v\n", command)
 	switch command.command_type {
 		fmt.Fprintln(ctx.stdout, strings.Join(command.args, " "))
 		return 0
 
+	case CommandTypeBuiltinCd:
+		switch len(command.args) {
+		case 0:
+			home_path := os.Getenv("HOME")
+			if len(home_path) == 0 {
+				fmt.Fprintln(ctx.stderr, "error: $HOME environment variable is empty or unset")
+				return 4
+			}
+			return bltnChdir(ctx, home_path)
+		case 1:
+			if command.args[0] == "~" {
+				home_path := os.Getenv("HOME")
+				if len(home_path) == 0 {
+					fmt.Fprintln(ctx.stderr, "error: $HOME environment variable is empty or unset")
+					return 4
+				}
+				return bltnChdir(ctx, home_path)
+			}
+			return bltnChdir(ctx, command.args[0])
+		default:
+			fmt.Fprintln(ctx.stderr, "usage: cd PATH")
+			return 2
+		}
+
 	case CommandTypeBuiltinPwd:
 		if len(command.args) != 0 {
 			fmt.Fprintln(ctx.stderr, "usage: pwd")