shell: 循环日期+1

start_date='20201201'
end_date='20201221'
while [ "${start_date}" -le "${end_date}" ];
do
    start_date=`date -d "$start_date" +%Y-%m-%d`
    echo ${start_date}
    start_date=$(date -d "$start_date+1days" +%Y%m%d)
done