bash_loop_seq

#!/bin/bash

from_dt=${$1:-2018-03-01}
end_dt=${$2:-2018-03-08}

dt_arr=(01 08 15 22)

for m in $(seq -f "%02g" 3 12)
do
  for d in ${dt_arr[*]}
  do
    end_dt="2018-$m-$d"
    echo $from_dt $end_dt
    from_dt=$end_dt
  done
done

3월부터 12월까지, 각 월의 [1일, 8일, 15일, 22일]에 대해서, from_dt, end_dt를 만들어준다.
매 월 딱 4일씩만 비교하는 구조라서 정확히 7일로 떨어지진 않음.

2018-03-01 2018-03-01
2018-03-01 2018-03-08
2018-03-08 2018-03-15
2018-03-15 2018-03-22
2018-03-22 2018-04-01
...
2018-12-01 2018-12-08
2018-12-08 2018-12-15
2018-12-15 2018-12-22