Bash script problem to mount

Shell Programming and Scripting
1 
#!/bin/bash

    userid=$(id -u)
    options="rw,exec,users,uid="${userid}",gid="${userid}""

    while [ 1 -gt 0 ];do
        echo -n "Enter device you want to mount (Example: /dev/sda11): "
        read device
        echo "Selected device: ${device}"

        if [ -e "${device}" ];then
            echo -n "Enter mount point (Example: /media/sda11): "
            read mountpoint
            echo "Select mountpoint: "${mountpoint}""

            if [ ! -e "${mountpoint}" ];then
                echo "${mountpoint}" does not exist"
                echo "Creating mountpoint "${mountpoint}"
                mkdir "${mountpoint}"
            fi

            su -c 'mount -t vfat -o "${options}" "${device}" "${mountpoint}"'
            echo ""${device}" is mounted in "${mountpoint}""
        fi    

        echo -n "Do you want to continue?(yes/no): "
        read val

        if [ "${val}" == "no" ];then
            break
        fi
    done
    echo "Done"

Error:

mount: mount point  does not exist

Why?

2

Try:

su -c "mount -t vfat -o ${options} ${device} ${mountpoint}"
3

Why doesn't work?

su -c 'mount -t vfat -o ${options} ${device} ${mountpoint}'
4

If you use single quotes, then the variables ${options}, ${device} ${mountpoint} never get evaluated...