test

sudo apt install inotify-tools

sudo dd if=/dev/zero of=/usbdisk.img bs=1M count=128

fdisk new primary partition type b


fdisk -lu /usbdisk.img   #=> offset

losetup -o $((2048*512)) /dev/loop0 /usbdisk.img
mkfs -t vfat -v /dev/loop0 -n raspberry
losetup -d /dev/loop0


sudo modprobe g_mass_storage file=/usbdisk.img stall=0 removable=1 iSerialNumber=1234567890
(modinfo g_mass_storage)

mount -o loop,ro,offset=$((2048*512)) -t vfat /usbdisk.img /mnt

umount /mnt

sudo modprobe -r g_mass_storage



echo "dtoverlay=dwc2" >> /boot/config.txt
echo "g_mass_storage file=/usbdisk.img stall=0 removable=1 iSerialNumber=1234567890" >> /etc/modules


script:
#!/bin/bash
set -e -u

[ -f /usr/bin/inotifywait ] || apt -y install inotify-tools
[ -f /usr/bin/fim ] || apt -y install fim

#tvservice -p
# tvservice -e "DMT 82"
# fbset -g 1920 1080 1920 1080 16

echo "18" > /sys/class/gpio/export || echo "gpio already set up"
echo "23" > /sys/class/gpio/export || echo "gpio already set up"
sleep 1
echo "out" > /sys/class/gpio/gpio18/direction
echo "0" > /sys/class/gpio/gpio18/value
echo "out" > /sys/class/gpio/gpio23/direction
echo "0" > /sys/class/gpio/gpio23/value

declare -g latest=""

getnewestfile() {
    #unset -v latest
    shopt -s nullglob
    for f in /mnt/*; do
        [[ "$f" -nt "$latest" ]] && latest="$f"
    done
    echo "$latest"
}

cleanup () {
    echo "0" > /sys/class/gpio/gpio18/value
    echo "0" > /sys/class/gpio/gpio23/value
    killall fim
    umount /mnt || echo ""
    modprobe -r g_mass_storage || echo ""
    exit
}

trap cleanup INT TERM EXIT

modprobe g_mass_storage file=/usbdisk.img stall=0 removable=1 iSerialNumber=1234567890

echo "1" > /sys/class/gpio/gpio18/value

inotifywait -mq -e modify --format %w%f /usbdisk.img | while read FILE
do
    echo "1" > /sys/class/gpio/gpio23/value
    echo "new screenshot"
    mount -o loop,ro,offset=$((2048*512)) -t vfat /usbdisk.img /mnt
    latest=$(getnewestfile)
    if [ -n "$latest" ] && [ -f "$latest" ]; then
        killall fim
        cp "$latest" /latestimage
        fim -d /dev/fb0 -T 1 --autozoom --quiet /latestimage &
    fi
    umount /mnt
    echo "0" > /sys/class/gpio/gpio23/value
done





examples:

FILE=/usbdisk.img
mkdir -p ${FILE/img/d}
mount -o loop,offset=$((2048*512)) -t vfat $FILE ${FILE/img/d}


inotifywait -mq -e close_write --format %w%f /usbdisk.img | while read FILE
do
    echo "Die Datei $FILE wurde modifiziert."
    echo "1" > /sys/class/gpio/gpio23/value
    mount -o loop,ro,offset=$((2048*512)) -t vfat /usbdisk.img /mnt

    umount /mnt
    echo "0" > /sys/class/gpio/gpio23/value
done


while inotifywait -e modify /usbdisk.img
do
    ...
done



ToDo:

inotify only once!!
recreate image on boot

readonly-root


python3-pyinotify