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DEFAULT_DAYS_BACK="0";
DAYS_BACK=${1:-$DEFAULT_DAYS_BACK};
PROCESSING_DATE=`date --date="$DAYS_BACK day ago" +%Y_%m_%d`;
The above code will populate variable PROCESSING_DATE with the date of the day the was DAYS_BACK ago using the format YYYY_MM_DD.
The value for DAYS_BACK is filled by the first argument of the script/function $1. In case the user did not provide an argument, we have the variable DEFAULT_DAYS_BACK that will be used as the default value.