#!/bin/bash
set -e

print_usage() {
    cat 1>&2 <<EOF
Usage: $0 rootfs-directory image
Package the contents of a directory into an ext4-based binary image file.
EOF
}

# Process options
#
while [ $# -gt 2 ]
do
    case "$1" in
	--help|-h)
	    print_usage
	    exit 0
	    ;;
	*)
	    echo "Unknown option '$1'" 1>&2
	    exit 1
    esac
    shift
done

# Process last argument, the root file system location
if [ -z "$1" ] || [ -z "$2" ] || [ "$1" = -h ] || [ "$1" = --help ]; then
    print_usage
    exit 1
fi
if [ "$EUID" -ne 0 ]; then
    echo "This must be run as root." 1>&2
    exit 1
fi

# Directory to package
rootfs="$1"

# Binary image file
image="$2"

# Number of 512-byte sectors reserved at the beginning of the image
# (usually 1M for e.g. a bootloader)
table_sectors=$(expr 1 \* 1024 \* 1024 \/ 512)

# Calculate size of the rootfs directory in KB
rootfs_size=$(expr `du -s "${rootfs}" | awk '{ print $1 }'`)

# The root partition is EXT4
# This means more space than the actual used space of the rootfs is used.
# As overhead for journaling and reserved blocks 25% are added.
rootfs_sectors=$(expr $(expr ${rootfs_size} + ${rootfs_size} \/ 100 \* 25) \* 1024 \/ 512)

# Calculate required image size in 512 Byte sectors
image_sectors=$(expr ${table_sectors} + ${rootfs_sectors})

# Initialize image file
dd if=/dev/zero of="$image" bs=512 count=${table_sectors}
dd if=/dev/zero of="$image" bs=512 count=0 seek=${image_sectors}

# Write partition table
sfdisk "$image" <<EOF
${table_sectors},${rootfs_sectors},L,*
EOF

# Setup temporary loop devices
image_loop="$(losetup -o 1M -f --show $image)"

mkfs.ext4 -O^64bit "$image_loop"

cleanup() {
    losetup -d "$image_loop" 2> /dev/null
    trap - 0 1 2 3 6
}
trap cleanup 0 1 2 3 6

# Mount the temporary loop devices
mount_dir=$(mktemp -d)
mount "$image_loop" "$mount_dir"

# Copy all files from the chroot to the loop device mount point directory
rsync -a "$rootfs/" "$mount_dir"
umount "$mount_dir"
cleanup