In BASH/ksh for basic validation of startDate
in yyyMMDD
you can use:
[[ "$startDate" == [0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9] ]] && echo "valid"
To get next day from $startDate
you can use:
nextDay=`date '+%Y%m%d' -d "$startDate next day"`