Implement #EI0

app/main.go

 	CommandTypeBuiltinExit
 	CommandTypeBuiltinEcho
 	CommandTypeBuiltinType
+	CommandTypeBuiltinPwd
 )
 
 func getCommand(ctx *Context) (Command, error) {
 	if tokens[0] == "exit" {
 		return makeCommandBuiltin(CommandTypeBuiltinExit, tokens), nil
 	}
+	if tokens[0] == "pwd" {
+		return makeCommandBuiltin(CommandTypeBuiltinPwd, tokens), nil
+	}
 	if tokens[0] == "echo" {
 		return makeCommandBuiltin(CommandTypeBuiltinEcho, tokens), nil
 	}
 		os.Exit(0)
 	case CommandTypeBuiltinEcho:
 		fmt.Fprintln(ctx.stdout, strings.Join(command.tokens[1:], " "))
+	case CommandTypeBuiltinPwd:
+		if len(command.tokens) != 1 {
+			fmt.Fprintln(ctx.stderr, "usage: pwd")
+			return 2
+		}
+		cwd_path, err := os.Getwd()
+		if err != nil {
+			panic("error getting current working directory")
+		}
+		fmt.Fprintln(ctx.stdout, cwd_path)
 	case CommandTypeBuiltinType:
 		if len(command.tokens) != 2 {
 			fmt.Fprintln(ctx.stderr, "usage: type COMMAND")