O -r faz a mesma coisa para ls como tac para qualquer comando que precise de ordenação reversa de arquivos.
Você pode simplesmente escrever
ls -ltr
Da página man:
-lList in long format. If the output is to a terminal, a total sum for all the file sizes is output on a line before the long listing.-rReverse the order of the sort to get reverse lexicographical order or the oldest entries first (or largest files last, if combined with sort by size.-tSort by time modified (most recently modified first) before sorting the operands by lexicographical order.