[alpap]

при таком содержимом

1.txt

1.1.1.1
3.3.3.3

2.bat

@Echo Off

Set ""
Set "var=4.4.4.4"
pause

и коде:

Код:

@echo off
set "f1=1.txt"
set "f2=2.bat"
set ns=4

<"%f1%" (set /p=& set /p s2=)
<"%f2%">+ (for /f "tokens=1* delims=[]" %%a in ('find /n /v ""') do @echo [%%a]|>nul find "[%ns%]" && (
  echo Set "var=%s2%"
 ) || (
  <nul set /p="%%b"& echo:
 )
)& >nul move+ "%f2%"
pause

получим вывод (новое содержимое)

2.bat

@Echo Off

Set ""
Set "var=3.3.3.3"
pause