pull

#! /bin/sh


case "$1" in
    ( -h | --help )
        printf "%s\n" \
            "pull: Fetch an image from DockerHub into a directory." \
            "" \
            "Usage:" \
            "  pull [-h|--help]     Show this help." \
            "  pull dir img         Fetch img into dir."
        exit;;
esac


#   Functions.

err () { printf "${0##*/}: \e[31mError:\e[0m %s\n" "$*" >&2; exit 1; }

_get_manifest () {
    curl -f -s \
         -H "Authorization: Bearer $3" \
         -H "Accept: application/vnd.docker.distribution.manifest.v2+json" \
         "https://index.docker.io/v2/$1/manifests/$2"
}


_pull_layer () {
    echo "Downloading '$1'."
    curl -f -# -OL \
         -H "Authorization: Bearer $3" \
         -H "Accept: application/vnd.docker.image.rootfs.diff.tar.gzip" \
         "https://index.docker.io/v2/$2/blobs/$1"
}


_pull_layers () {
    jq -r .layers[].digest | while read d; do
        _pull_layer "$d" "$1" "$2"
    done
}


_pull () {
    cd "$1"
    _get_manifest "$2" "$3" "$4" | _pull_layers "$2" "$4"

    for f in *; do
        tar --force-local -xf "$f"
        rm "$f"
    done
}


#   Process the request.

test $# -eq 2 ||
    err "wrong number of arguments."

tag="${2##*:}"
img="${2%%:*}"
owner="${img%%/*}"
out="$1"

test "$img" ||
    err "No image provided."

mkdir -p "$out"

test -z "$tag" -o "$tag" = "$img" && tag=latest
test -z "$owner" -o "$owner" = "$img" && img="library/$img"

echo "Pulling $img:$tag..."

until token=$(curl -sf "https://auth.docker.io/token?service=registry.docker.io&scope=repository:$img:pull" | jq -r .token); do :; done

_pull "$out" "$img" "$tag" "$token"