set "fa=a.txt"

set "fb=b.txt

set "fc=c.txt"

echo. >"%fc%"

echo. >"%fc%"

(for /f "delims=" %%a in ('type "%fa%"') do (

    set /p line=

    set "str=%%a"

    set n=0

    setlocal enabledelayedexpansion

    for /l %%b in (0 1 8100) do (

        if "!str:~%%b,1!" neq "" (

            set /a n+=1

        )

    )

    set /a "m=!random! %% (n+1)"

    echo;长度!n!   随机插入位置点!m!

    for %%c in (!m!) do >>"%fc%" echo;!str:~,%%c!!line!!str:~%%c!

    endlocal

))<"%fb%"