ComSpec (or %COMSPEC% ) is one of the environment variables used in MS-DOS and Microsoft Windows, which normally points to the command line interpreter, ..."
-- en.wikipedia.org/wiki/ComSpec
If test1.txt is:
[tt]abc[/tt] 'no return character
and test2.txt is:
[tt]def[/tt]
test3.txt will be:
[tt]abcdef[/tt] 'eof
If test1.txt is:
[tt]abc[/tt] 'with a return character
test3.txt will be:
[tt]abc
def[/tt] 'eof