poter

用findstr最简单，一条命令可以搞定

用循环就麻烦很多
@echo off
setlocal enableDelayedExpansion
for /f "tokens=*" %%k in (b.txt) do (
        set /a a+=1
)

for /f "tokens=*" %%i in (a.txt) do (
        for /f "tokens=*" %%j in (b.txt) do (
                if not "%%i"=="%%j" set /a b+=1
                if "!b!"=="!a!" echo %%i
        )
        set b=
)