use_nix() {
  local path="$(nix-instantiate --find-file nixpkgs)"

  if [ -f "${path}/.version-suffix" ]; then
    local version="$(< $path/.version-suffix)"
  elif [ -f "${path}/.git" ]; then
    local version="$(< $(< ${path}/.git/HEAD))"
  fi

  local cache=".direnv/cache-${version:-unknown}"

  local update_drv=0
  if [[ ! -e "$cache" ]] || \
    [[ "$HOME/.direnvrc" -nt "$cache" ]] || \
    [[ .envrc -nt "$cache" ]] || \
    [[ default.nix -nt "$cache" ]] || \
    [[ shell.nix -nt "$cache" ]];
  then
    [ -d .direnv ] || mkdir .direnv
    nix-shell --show-trace --pure "$@" --run "\"$direnv\" dump bash" > "$cache"
    update_drv=1
  else
    log_status using cached derivation
  fi
  local term_backup=$TERM path_backup=$PATH
  if ! [ -z ${TMPDIR+x} ]; then
    local tmp_backup=$TMPDIR
  fi

  eval "$(< $cache)"
  export PATH=$PATH:$path_backup TERM=$term_backup TMPDIR=$tmp_backup
  if ! [ -z ${tmp_backup+x} ]; then
    export TMPDIR=${tmp_backup}
  else
    unset TMPDIR
  fi

  # `nix-shell --pure` sets invalid ssl certificate paths
  if [ "${SSL_CERT_FILE:-}" = /no-cert-file.crt ]; then
    unset SSL_CERT_FILE
  fi
  if [ "${NIX_SSL_CERT_FILE:-}" = /no-cert-file.crt ]; then
    unset NIX_SSL_CERT_FILE
  fi

  # This part is based on https://discourse.nixos.org/t/what-is-the-best-dev-workflow-around-nix-shell/418/4
  if [ "$out" ] && (( $update_drv )); then
    local drv_link=".direnv/drv"
    local drv="$(nix show-derivation $out | grep -E -o -m1 '/nix/store/.*.drv')"
    local stripped_pwd=${PWD/\//}
    local escaped_pwd=${stripped_pwd//-/--}
    local escaped_pwd=${escaped_pwd//\//-}
    ln -fs "$drv" "$drv_link"
    ln -fs "$PWD/$drv_link" "/nix/var/nix/gcroots/per-user/$LOGNAME/$escaped_pwd"
    log_status renewed cache and derivation link
  fi

  if [[ $# = 0 ]]; then
    watch_file default.nix
    watch_file shell.nix
  fi
}