怎么实现写个批处理文件,修改指定文件的内容?谢谢大神!!!

@echo off
set a=x.txt
set b=y.txt
set x=xxx
set y=yyy
pushd d:\
if exist %a% goto ab
set c=%a%
set a=%b%
set b=%c%
if exist %% goto ab
echo %b% %a%都不存在
goto :end
:ab
(for /f tokens=* %%i in ('type %a%') do (
set s=%%i
call set s=!s:%x%=%y%!
echo !s!))%b%
del %a%
:end
puase