Bash script to loop by date.

date_loop.sh

#!/bin/bash
DATE_S="20201226"
DATE_E="20210105"

# Example-1
for (( dt=$DATE_S ; $dt < $DATE_E ; dt=`date -d "$dt 1 day" '+%Y%m%d'` )) ; do
  echo $dt
done

echo "---"

# Example-2
dt=$DATE_S
while [[ $dt < $DATE_E ]] ; do
  echo $dt
  dt=`date -d "$dt 1 day" '+%Y%m%d'`
done