Вот пример того, что я использовал на основе одного из других ответов
@echo off
REM SOME CODE
set __ERROR_LOG=c:\errors.txt
REM set __IPADDRESS=x.x.x.x
REM Test a variable
if not defined __IPADDRESS (
REM Call function with some data and terminate
call :TEE %DATE%,%TIME%,IP ADDRESS IS NOT DEFINED
goto :EOF
)
REM If test happens to be successful, TEE out a message and end script.
call :TEE Script Ended Successful
goto :EOF
REM THE TEE FUNCTION
:TEE
for /f "tokens=*" %%Z in ("%*") do (
> CON ECHO.%%Z
>> "%__ERROR_LOG%" ECHO.%%Z
goto :EOF
)