I am 99.999% certain that the answer is "no", but I still hope that the answer might be
"yes" — is it possible to use \immediate \openout and \immediate \write in such a way as
to append to an existing file rather than overwriting it ? If not, is there a more efficient way
of achieving this th
You could switch to luatex then append from Lua.
Failing that, more efficient than repeatedly reading and copying in tex
way would be to not append but write a new file each time file-1.x
file-2.x ... then concat them in your operating system concat in a nix
shell, or copy*.x > realfile.txt in
