$ echo {0..19}
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
$ echo {00..19}
00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19
$echo {A..D}
A B C D
$ echo `seq 0 19`
$ perl -le 'print join(" ", "00".."20")' 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20
(the second doesn't work on older versions of bash, like macos)
$ echo {0..19}
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
$ echo {00..19}
00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19