create-qemu-img.sh

#!/bin/bash
#
# Creates a multi-partition QEMU disk image from our buildroot outputs and embeds grub2
#
# TODO: Better error handling
# TODO: Better args (grub.cfg path is assumed)
#

DEBUG=1
IMAGE_NAME="dhc.img"
IMAGE_SIZE="1G" # 100M used for /boot

function log_()
{
    echo "[$(basename "$0")] $*"
}

(( DEBUG )) && {
    KERNEL="${1:-./initrd-buildroot-2017.08/output/images/bzImage}"
    log_ "Debug: Using KERNEL = $KERNEL"
    INITRD="${2:-./initrd-buildroot-2017.08/output/images/rootfs.cpio.gz}"
    log_ "Debug: Using INITRD = $INITRD"
    ROOTFS="${3:-./rootfs-buildroot-2017.08/output/images/rootfs.tar.bz2}"
    log_ "Debug: Using ROOTFS = $ROOTFS"
}

[[ -e $KERNEL ]] || {
    log_ "$KERNEL: File not found"
    exit 1
}

[[ -e $INITRD ]] || {
    log_ "$INITRD: File not found"
    exit 1
}

[[ -e $ROOTFS ]] || {
    log_ "$ROOTFS: File not found"
    exit 1
}

[[ -e ./grub.cfg ]] || {
    log_ "./grub.cfg: File not found"
    exit 1
}

function atexit()
{
    [[ -d $MOUNT2 ]] && sudo umount "$MOUNT2" &> /dev/null
    [[ -d $MOUNT1 ]] && sudo umount "$MOUNT1" &> /dev/null
    sudo kpartx -d "$LOOPBACK" &> /dev/null
    sudo losetup -d "$LOOPBACK" &> /dev/null
    sudo qemu-nbd -d "$NBD" &> /dev/null
}

function error()
{
    log_ "Error: $*"
    atexit
    exit 1
}

function check_commands()
{
    for comm in qemu-img qemu-nbd losetup kpartx parted mkfs.ext2 mkfs.ext4 grub-install ;
    do
        command -v "$comm" &> /dev/null || {
            log_ "Command '$comm' is not available"
            exit 1
        }
    done
}

function find_next_nbd()
{
    for dev in /sys/class/block/nbd*;
    do
        local size
        size="$(cat "$dev"/size)"

        if (( size == 0 ));
        then
            printf "%s" "/dev/nbd${dev: -1}"
            return
        fi
    done

    error "No available nbd devices"
}

function find_next_loopback()
{
    sudo losetup -f &> /dev/null || error "No available loopback devices"
    printf "%s" "$(sudo losetup -f)"
}

#####

if [[ -e "$IMAGE_NAME" ]] ; then
    error "File '$IMAGE_NAME' already exists. Exiting to avoid overwriting!"
fi

# init
check_commands
sudo -v || exit
atexit
sudo modprobe nbd max_part=16

# vars
NBD="$(find_next_nbd)"
LOOPBACK="$(find_next_loopback)"
LOOPBACK_N="${LOOPBACK: -1}" # the space before -1 is very important
MAPPER1="/dev/mapper/loop${LOOPBACK_N}p1"
MAPPER2="/dev/mapper/loop${LOOPBACK_N}p2"

(( DEBUG )) && {
    log_ "Debug: NBD=$NBD"
    log_ "Debug: LOOPBACK=$LOOPBACK"
    log_ "Debug: LOOPBACK_N=$LOOPBACK_N"
    log_ "Debug: MAPPER2=$MAPPER1"
    log_ "Debug: MAPPER2=$MAPPER1"
}

export NBD
export LOOPBACK
export MAPPER1
export MAPPER2

#####

qemu-img create -f qcow2 "$IMAGE_NAME" "$IMAGE_SIZE" || error "create image"
log_ "[+] Created image"

sudo qemu-nbd -c "$NBD" "$IMAGE_NAME" || error "mount nbd"
log_ "[+] Bound to $NBD"

sudo parted --script "$NBD"  \
    mklabel msdos               \
    mkpart primary ext2 1M 100M \
    mkpart primary ext4 100M "$IMAGE_SIZE" || error "format"
log_ "[+] Partitioned via parted"
sleep 3

sudo losetup "$LOOPBACK" "$NBD" || error "losetup"
log_ "[+] losetup on $LOOPBACK"
sleep 3

sudo kpartx -a "$LOOPBACK" || error "kpartx"
sleep 3
[[ -e "$MAPPER1" ]] || error "mapper 1 doesnt exist"
[[ -e "$MAPPER2" ]] || error "mapper 2 doesnt exist"
log_ "[+] kpartx"

MOUNT1=$(mktemp -d)
MOUNT2="$MOUNT1/boot"
export MOUNT1
export MOUNT2
log_ "[+] Created mount dirs ($MOUNT1) ($MOUNT2)"

log_ "[...] Starting format..."
sudo mkfs.ext2 "$MAPPER1" || error "mkfs.ext2"
sudo mkfs.ext4 "$MAPPER2" || error "mkfs.ext4"
log_ "[+] Formatted in ext2 + ext4..."

sudo mount "$MAPPER2" "$MOUNT1" || error "Failed to mount ext4 partition"
log_ "[+] Mounted $MAPPER2 to $MOUNT1"
sudo mkdir -p "$MOUNT2" # /boot not not exist

sudo mount "$MAPPER1" "$MOUNT2" || error "Failed to mount ext2 partition"
log_ "[+] Mounted $MAPPER1 to $MOUNT2"
log_ "[+] Mounted loopback devices"

sudo cp "$KERNEL" "$MOUNT2" || error "Failed to copy kernel"
log_ "[+] Copied kernel"
sudo cp "$INITRD" "$MOUNT2/initrd.cpio.gz" || error "Failed to copy initrd"
log_ "[+] Copied initrd"
sudo tar -xjf "$ROOTFS"  -C "$MOUNT1" || error "Failed to extract rootfs"
log_ "[+] Copied rootfs"

sudo grub-install --target=i386-pc --boot-directory="$(readlink -f "$MOUNT2")" "$LOOPBACK" || error "Failed to install grub"
log_ "[+] Grub installed"

sudo cp ./grub.cfg "$MOUNT2/grub/grub.cfg" || error "Failed to write grub config"

sudo umount "$MOUNT2" || error "Failed to unmount loop2"
log_ "[+] Unmounted MOUNT2"

sudo umount "$MOUNT1" || error "Failed to unmount loop1"
log_ "[+] Unmounted MOUNT1"
rm -r "$MOUNT1" # remove this dir as we no longer need it

atexit

exit 0

At line 120:

    log_ "Debug: MAPPER2=$MAPPER1"
    log_ "Debug: MAPPER2=$MAPPER1"

Numbers are swapped.
Great script btw.

At line 112:

LOOPBACK_N="${LOOPBACK: -1}" # the space before -1 is very important

Should be replaced with:

LOOPBACK_N="${LOOPBACK//[!0-9]/}"

... to support loop devices started from 10 and above.
Cutting only one digit doesn't work.

Excellent script, immensely helpful in resolving my issues building disk images.