These instructions are not for a live install, but rather an installation of Debian to USB flash media.

An added bonus here is that we’re going to first prepare our Debian chroot inside an img file that we mount as a loopback device.

We can then use that img file as a generic base image that we can deploy to a large number of USB drives.

Why would you want to do this? Well, for one, the img file helps reduce some mess and gives you an easy to manage file that can be re-deployed again and again. This may also be ideal for creating a generic image that will be reused on multiple machines. Like you might do at a cybercafe where Debian is re-installed at each boot, or maybe for a classroom.

Thanks to Mike McCabe for the idea and for most of the steps detailed below.

See other related articles here:

Warning: I have highlighted all the places you should be in the chroot environment. Be careful! Running some of these commands on your local environment instead of in the chroot can cause issues. 

sudo apt-get install \
    debootstrap

Create the image file and format it.

fallocate -l 2G debian.img
echo -e "o\nn\np\n1\n\n\nw" | sudo fdisk debian.img
echo -e "a\nw\n" | sudo fdisk debian.img

Take note of what is returned by the following command. For me, the loopback device is /dev/loop0, but it may vary for you. Update the instructions below accordingly if your loopback device address differs. 

sudo losetup --partscan --show --find debian.img

Format the linux partition of our loopback device, much like we would do for a real physical device. 

sudo mkfs.ext4 /dev/loop0p1

Mount the partition.

sudo mkdir -p /mnt/debian
sudo mount /dev/loop0p1 /mnt/debian

Bootstrap the chroot.

sudo debootstrap \
    --arch=i386 \
    --variant=minbase \
    stretch /mnt/debian http://ftp.us.debian.org/debian/

Mount special devices to the chroot. This will be important for installing grub later. 

sudo mount -t proc /proc /mnt/debian/proc

sudo mount -t sysfs /sys /mnt/debian/sys

sudo mount -o bind /dev /mnt/debian/dev

Change root to the chroot environment.

sudo chroot /mnt/debian

chroot

These packages are required at a minimum, but add more as needed.

When prompted to install grub to a device, do NOT install it. Simply hit “Enter” and choose not to install it to any devices. 

apt-get update && \
apt-get install --no-install-recommends \
    linux-image-586 systemd-sysv \
	grub2-common grub-pc

chroot

We want fstab to mount / based on the disk label, and not a UUID or named disk like /dev/sda. That’s because, depending on the machine, the USB drive may not be /dev/sda, and also because the UUID will vary depending on where the img file is deployed. 

echo "LABEL=DEBUSB / ext4 defaults 0 1" > /etc/fstab

chroot 

passwd root

chroot 

grub-install \
    --target=i386-pc \
    --boot-directory=/boot \
    --force-file-id \
    --skip-fs-probe \
    --recheck /dev/loop0

chroot 

exit

Edit the grub.cfg file in the img.

sudo nano /mnt/debian/boot/grub/grub.cfg

Paste this content into the grub.cfg file.

# grub.cfg
set default="0"
set timeout=10

menuentry "Debian" {
    linux /vmlinuz root=/dev/disk/by-label/DEBUSB quiet
    initrd /initrd.img
}

Label the image partition with the same name we used for fstab above. 

sudo e2label /dev/loop0p1 DEBUSB

Set the hostname for the img.

echo "debian-usb" | sudo tee /mnt/debian/etc/hostname

Clean up special devices.

sudo umount /mnt/debian/{dev,sys,proc}

Unmount the loop device.

sudo umount /mnt/debian

Unmount the img.

sudo losetup -d /dev/loop0

Now you have a generic Debian installation that you can deploy to multiple devices like so. 

dd if=debian.img of=/dev/sdz

Citations