Run command after Docker image update, but nothing to do if no update

run_after_update.sh

#!/bin/sh

script=$(basename $0)
image=$1
arg=$@

function current() {
    local a=$(docker inspect ${image} -f '{{index .RepoDigests 0}}')
    echo ${a#*@}
}

function latest() {
    docker pull ${image} | grep Digest | awk '{print $2}'
}

function usage() {
    echo "Usage: ${script} name[:tag|@digest] [-- cmd [arguments]]"
}

function cmd() {
    echo ${arg#*--}
}

function main() {
    if [[ "$image" == "" ]]; then
        usage
        exit 1
    fi

    # Get current first
    currentHash=$(current)
    # Get latest second
    latestHash=$(latest)

    echo "OLD:[$latestHash]"
    echo "NEW:[$currentHash]"

    if [[ "$latestHash" != "$currentHash" ]]; then
        exec $(cmd)
    fi
}

main