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:
-l
List 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.-r
Reverse 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.-t
Sort by time modified (most recently modified first) before sorting the operands by lexicographical order.