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@ECHO OFF
SETLOCAL
set "var1=Hello world"
Echo var1 before: %var1%
call:myDosFunc var1 
Echo var1 after : %var1%
goto :eof

:myDosFunc    -- passing a variable by reference, and changing value
echo   Notice that a variable passed by reference does NOT echo the value, and it echos the variable name instead.

CALL SET "arg2=%%%~1%%"
echo %arg2%
echo   Arg1 variable name = '%~1' and Arg2 value = '%~2'
set "%~1=Good bye world!!!"
goto :eof