Shell takes the month within a certain time range

#!/bin/sh
if [ -n "$1" ] ;then
  do_date=$1
else
  do_date=`date -d "-1 day" +%Y-%m-%d`
fi
 
end_date="2018-01-01"

>./month_num.txt
while [[ $do_date > $end_date ]]
do
echo ${do_date:0:7} >>./month_num.txt
do_date=`date -d "-1 month $do_date" +%Y-%m-%d`
done