How to determine an RPi kernel version and build without booting it

To figure out the kernel version and build without booting it, e.g. to install matching device drivers during an automated image build in something like CustoPiZer, use something like this:

function version_and_build_for_kernelimg() {
    kernelimg=$1

    # uncompressed kernel?
    output=$(strings $kernelimg | grep 'Linux version' || echo)

    if [ -z "$output" ]; then
        # compressed kernel, needs more work, see https://raspberrypi.stackexchange.com/a/108107
        pos=$(LC_ALL=C grep -P -a -b -m 1 --only-matching '\x1f\x8b\x08' $kernelimg | cut -f 1 -d :)
        dd if=$kernelimg of=kernel.gz skip=$pos iflag=skip_bytes
        output=$(gzip --decompress --stdout kernel.gz | strings | grep 'Linux version' || echo)
    fi

    version=$(echo $output | awk '{print $3}' | tr -d '+')
    build=$(echo $output | awk -F"#" '{print $NF}' | awk '{print $1}')

    if [[ -n "$version" && -n "$build" ]]; then
        echo "Version: $kernel"
        echo "Build: $build"
    else
        echo
        echo "Cannot determine kernel version and build number for $kernelimg"
    fi
}

Note that this has only been tested with kernels on RaspberryPi OS images, YMMV.