message: Fix shellcheck installation on macOS
  The shellcheck download verification step was relying on `sha256sum`,
  which is not present in the macOS default install.
diff: @@ -20,6 +20,17 @@ readonly VERSION='v0.7.1'
  DOWNLOAD_DIR="$(dirname "$0")/download"
  readonly DOWNLOAD_DIR
  +# `sha256sum` is part of GNU coreutils but is not available in macOS.
  +# macOS does have `shasum` (a Perl script designed to match the behavior
  +# of `sha256sum`) in the default install.
  +function sha256wrapper() {
  + if command -v sha256sum &> /dev/null; then
  + sha256sum "$@"
  + else
  + shasum -a 256 "$@"
  + fi
  +}
  +
  declare file="shellcheck-${VERSION}" # Name of the file to download
  declare cmd="${DOWNLOAD_DIR}/shellcheck-${VERSION}" # Path to the executable
  case "$(uname -s)" in
  @@ -36,7 +47,7 @@ if [[ ! -s "${cmd}" ]]; then
  curl --location --fail --output "${DOWNLOAD_DIR}/${file}" "${url}"
  pushd "${DOWNLOAD_DIR}"
  - sha256sum --check --ignore-missing ../hashes.sha256
  + sha256wrapper --check --ignore-missing ../hashes.sha256
  if [[ "${file}" == *'.tar.xz' ]]; then
  tar xf "${file}"
  else
  