shrink-image

#!/bin/bash

set -e
shopt -s lastpipe extglob
set -o pipefail

die() {
    echo "$@" >&2
    exit 1
}

[[ $# -ge 1 && $# -le 2 ]] || die "Usage: ${BASH_SOURCE} source.img [destination.img]"

image="$1"
[[ -f "$image" ]] || die "File not found: $image"
if [[ $# -ge 2 ]]; then
    newimage="$2"
else
    # remove suffix indicating image size
    if [[ "$image" == *-[1-9]gb.img ]]; then
        newimage="${image%-[1-9]gb.img}.img"
    else
        newimage="$image"
    fi
fi
[[ "$newimage" == "$image" || ! -f "$newimage" ]] || die "File exists: $newimage"

# determine start of rootfs partition
/sbin/sfdisk --dump /dev/stdin <"$image" | tail -n 1 | {
    read -rd,
    [[ "$REPLY" == '/dev/stdin1 : start='* ]] \
        || die "Not a single-partition image"
    start="${REPLY##*=}"
    (( start % 8 == 0 )) || die "Invalid partition start: $start"
    start=$(( start / 2 ))K
}

# rename image if requested
if [[ "$newimage" != "$image" ]]; then
    mv "$image" "$newimage"
    image="$newimage"
fi

# split off the part before the rootfs partition (partition table, bootloader)
head -c $start "$image" >"$image.header"
fallocate -c -l $start "$image"

# shrink filesystem to minimum size
/sbin/e2fsck -f -p "$image" || [[ $? == 1 ]]
/sbin/resize2fs -M "$image"

# determine size of filesystem
declare -A info
/sbin/dumpe2fs -h "$image" 2>/dev/null | grep -P ': *\d+$' | \
    while IFS=: read -r k v; do
        info["${k// /_}"]="${v##*( )}"
    done
size=$(( ${info[Block_size]:?} * ${info[Block_count]:?} / 1024 ))K

# truncate image file to filesystem size
truncate --no-create --size=$size "$image"

# reinsert the stuff before the rootfs partition (partition table, bootloader)
fallocate -i -l $start "$image"
cat "$image.header" 1<>"$image"
rm "$image.header"

# update partition table to match new filesystem size
echo ",$size" | /sbin/sfdisk -N 1 "$image"

# all done!  report image name and size
echo "$image  ($start + $size)"