#!/usr/bin/env bash
ARCHIVE="/var/userbak/$(date -I)-bak.tar.gz"
tar cfz "${ARCHIVE}" /home
echo "Backup stored at ${ARCHIVE}"

read -r -d $'\0' line < <(find /var/userbak/ -maxdepth 1 -type d -printf '%T@ %p\0' 2>/dev/null | sort -z -n)
OLDEST_FILE="${line#* }"

read -p "Do you want to delete ${OLDEST_FILE}? (y/N)" -n 1 -r
echo
if [[ $REPLY =~ ^[Yy]$ ]]
then
        rm "${OLDEST_FILE}"
        echo "Done"
fi

# Note: I'm not an expert in Bash, be careful when running this.