pull_image.sh

#!/bin/bash

set -e

scriptdir="$(cd "$(dirname "$0")"; pwd)"
script="${scriptdir%/}/$(basename "$0")"

readonly IMAGE_DIR="${HOME}/images/"

# escape docker tag for filename
escape() {
  for (( i = 0; i < "${#1}"; ++i )); do
    case "${1:i:1}" in
      ":"|"/")
        printf '%%%X' "'${1:i:1}"
        ;;
      *)
        printf '%c' "${1:i:1}"
        ;;
    esac
  done
}

pull_image() {
  mkdir -p "${IMAGE_DIR}"
  local repo="${1}"
  local tag="${2}"

  if [ -d "${IMAGE_DIR}/layers/$(escape "${repo}:${tag}")" ]; then
    # already exist
    return
  fi

  local tmpdir="$(mktemp -d)"
  trap "rm -rf '${tmpdir}'" RETURN

  local token="$(curl -sSf -L "https://auth.docker.io/token?service=registry.docker.io&scope=repository:${repo}:pull" | jq -rc .token)"
  curl -sSf -L --header "Authorization: Bearer ${token}" "https://registry-1.docker.io/v2/${repo}/manifests/${tag}" -o "${tmpdir}/manifest"

  local blobs=($(jq -rc '.fsLayers | .[].blobSum' "${tmpdir}/manifest"))
  local ids=($(jq -rc '.history | .[].v1Compatibility | fromjson | .id' "${tmpdir}/manifest"))
  local parents=($(jq -rc '.history | .[].v1Compatibility | fromjson | .parent' "${tmpdir}/manifest"))
  local sha256s=("${blobs[@]#sha256:}")

  for ((i = 0; i < ${#sha256s[@]}; ++i)); do
    if [ ! -f "${IMAGE_DIR}/blobs/${sha256s[i]}.tar-split.gz" ]; then
      mkdir -p "${IMAGE_DIR}/blobs/"
      curl -Sf -L --header "Authorization: Bearer ${token}" "https://registry-1.docker.io/v2/${repo}/blobs/sha256:${sha256s[i]}" -o "${IMAGE_DIR}/blobs/${sha256s[i]}.tar-split.gz"
    fi

    if [ ! -d "${IMAGE_DIR}/blobs/${sha256s[i]}" ]; then
      mkdir -p "${IMAGE_DIR}/blobs/${sha256s[i]}"
      tar -C "${IMAGE_DIR}/blobs/${sha256s[i]}" -zxf "${IMAGE_DIR}/blobs/${sha256s[i]}.tar-split.gz"
    fi

    mkdir -p "${IMAGE_DIR}/layers/${ids[i]}"
    if [ "${parents[i]}" != "null" ]; then
      ln -T -sf "../${parents[i]}" "${IMAGE_DIR}/layers/${ids[i]}/parent"
    fi
    ln -T -sf "../../blobs/${sha256s[i]}.tar-split.gz" "${IMAGE_DIR}/layers/${ids[i]}/blob.tar-split.gz"
    ln -T -sf "../../blobs/${sha256s[i]}" "${IMAGE_DIR}/layers/${ids[i]}/blob"
  done

  ln -T -sf "${ids[0]}" "${IMAGE_DIR}/layers/$(escape "${repo}:${tag}")"
}

if [ $# -lt 2 ]; then
  echo "Usage: $0 REPO TAG"
  exit 1
fi
pull_image "$1" "$2"