とりあえずこんなかんじで。sortコマンドのポジション指定は1-originなんだって。
$ ls file*.txt | sort -k 1.5 -n file0.txt file1.txt file2.txt file3.txt file4.txt file5.txt file6.txt file7.txt file8.txt file9.txt file10.txt file11.txt file12.txt file13.txt file14.txt file15.txt
-k, --key=POS1[,POS2]
start a key at POS1 (origin 1), end it at POS2 (default end of line)
