This is the problem:
set /a var1= %todo% & echo %linea%>>%exename%
set todo= %var1%'
What I want is that %var1% contain all the information above, and then pass it to %todo%
This is mostly that and the & is detected as if it were more than 1 command on a line.
When using the /a flag for arithmetic expressions that contain logical operations it is necessary to enclose the expression in double quotes ( "..." ):
set /a "var1=%todo% & %linea%>>%exename%"
set "todo=%var1%"
Here is a link with information and examples: link
To see:
The problem is that %var1% is equal to 0 because the elements & and >> block it.
Sorry to put the image, the code does not put me as such, but anyway here is:
@echo off
set exename= Ejemplo
:loop
set /p linea= Nueva linea (Escribe 0 para terminar.):
if %linea%== 0 goto guardar
set /a "var1=%todo% & %linea%>>%exename%"
set todo= %var1%
echo %var1%
pause
goto loop