Code:
$
$ (seq -s, 1 20; seq -s, 11 30; seq -s, 21 40) >data.txt
$
$ cat data.txt
1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20
11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30
21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40
$