[Sh] for date `start ~ end`

start=20200101
end=20201231

date=$start
while [ $date -le $end ]
do
    date=`date -d "$date+1 days" +%Y%m%d`
done

https://www.cyberciti.biz/faq/how-to-add-days-to-date-and-get-new-date-on-linux/

How to add days to date and get new date on Linux - nixCraft