To create an usb with multiple ISO on it

multiusb.sh

# Write dev path to your usb
myDEV="/dev/sdc"

# Write Label for your usb
myVOL="MultiBootUSB"


##### DO NOT EDIT BELOW #####

# Write 10MegaByte to clear formats on usb
sudo dd if=/dev/zero of="$myDEV" bs=1M count=10

# Create MBR format and create 1 partition with all available space
echo -en "o\nn\np\n1\n\n\nw\n" | sudo fdisk "$myDEV"

# Format partition with ex4
sudo mkfs.ext4 -j -O extent -L "$myVOL" "$myDEV"1

# Mount partition on /mnt
sudo mount "$myDEV"1 /mnt

# Take ownership of fs
sudo chown "$USER":"$USER" /mnt

# Install grub
sudo grub-install --target=i386-pc --force --removable --boot-directory=/mnt "$myDEV"

# Create grub.cfg
#sudo grub-mkconfig -o /mnt/grub/grub.cfg
sudo touch /mnt/grub/grub.cfg
sudo chmod 666 /mnt/grub/grub.cfg
sudo cat <<EOF > /mnt/grub/grub.cfg
insmod regexp
insmod all_video

for iso_with_device in /isos/*.iso; do

	# Remove the grub device name to get just the path to the iso relative to its root.
	regexp --set=iso_path '^\(.*\)(.*$)' "$iso_with_device"

	menuentry "loop boot $iso_with_device" "$iso_path" {
		iso_path="$2"
		export iso_path
		search --set=root --file "$iso_path"
		loopback loop "$iso_path"
		root=(loop)
		configfile /boot/grub/loopback.cfg
		loopback --delete loop
	}
done
# Thanks to Jordan Uggla (Jordan_U on irc.freenode.net)
# https://lists.gnu.org/archive/html/help-grub/2016-01/msg00065.html
EOF
sudo chmod 444 /mnt/grub/grub.cfg

# Create isos folder
sudo mkdir /mnt/isos
sudo chown "$USER":"$USER" /mnt/isos

# Umount /mnt
sudo umount /mnt