{
  bc,
  writeShellApplication,
}:
let
  deploy = writeShellApplication {
    name = "deploy";
    text = ''
      set -euo pipefail
      shopt -s lastpipe # allow cmd | readarray

      function die() { echo "error: $*" >&2; exit 1; }
      function show_help() {
        echo 'Usage: deploy [OPTIONS] <host,...> [ACTION]'
        echo "Builds, pushes and activates nixosConfigurations on target systems."
        echo ""
        echo 'ACTION:'
        echo '  switch          [default] Switch immediately to the new configuration and make it the boot default'
        echo '  boot            Make the configuration the new boot default'
        echo "  test            Activate the configuration but don't make it the boot default"
        echo "  dry-activate    Don't activate, just show what would be done"
        echo ""
        echo 'OPTIONS: [passed to nix build]'
      }

      function time_start() {
        T_START=$(date +%s.%N)
      }

      function time_next() {
        T_END=$(date +%s.%N)
        T_LAST=$(${bc}/bin/bc <<< "scale=1; ($T_END - $T_START)/1")
        T_START="$T_END"
      }

      USER_FLAKE_DIR=$(git rev-parse --show-toplevel 2>/dev/null || pwd) \
        || die "Could not determine current working directory. Something went very wrong."
      [[ -e "$USER_FLAKE_DIR/flake.nix" ]] \
        || die "Could not determine location of your project's flake.nix. Please run this at or below your main directory containing the flake.nix."
      cd "$USER_FLAKE_DIR"

      [[ $# -gt 0 ]] || {
        show_help
        exit 1
      }

      OPTIONS=()
      POSITIONAL_ARGS=()
      while [[ $# -gt 0 ]]; do
        case "$1" in
          "help"|"--help"|"-help"|"-h")
            show_help
            exit 1
            ;;

          -*) OPTIONS+=("$1") ;;
          *) POSITIONAL_ARGS+=("$1") ;;
        esac
        shift
      done

      [[ ''${#POSITIONAL_ARGS[@]} -ge 1 ]] \
        || die "Missing argument: <hosts...>"
      [[ ''${#POSITIONAL_ARGS[@]} -le 2 ]] \
        || die "Too many arguments given."

      tr , '\n' <<< "''${POSITIONAL_ARGS[0]}" | sort -u | readarray -t HOSTS
      ACTION="''${POSITIONAL_ARGS[1]-switch}"

      # Expand flake paths for hosts definitions
      declare -A TOPLEVEL_FLAKE_PATHS
      for host in "''${HOSTS[@]}"; do
        TOPLEVEL_FLAKE_PATHS["$host"]=".#nixosConfigurations.$host.config.system.build.toplevel"
      done

      time_start
      #echo "    Building 📦 ''${#TOPLEVEL_FLAKE_PATHS[*]} configuration(s)"
      #nix build --no-link "''${TOPLEVEL_FLAKE_PATHS[@]}" "''${OPTIONS[@]}" \
      #  || die "Failed to build derivations"
      #time_next
      #echo "       Built ✅ ''${#TOPLEVEL_FLAKE_PATHS[*]} configuration(s) in ''${T_LAST}s"

      # Get outputs of all derivations (should be cached)
      declare -A TOPLEVEL_STORE_PATHS
      for host in "''${HOSTS[@]}"; do
        toplevel="''${TOPLEVEL_FLAKE_PATHS["$host"]}"
        echo "    Building 📦 $host"
        TOPLEVEL_STORE_PATHS["$host"]=$(nix build --no-link --print-out-paths "''${OPTIONS[@]}" "$toplevel") \
          || die "Failed to get derivation path for $host from ''${TOPLEVEL_FLAKE_PATHS["$host"]}"
        time_next
        echo "       Built ✅ $host ''${TOPLEVEL_STORE_PATHS["$host"]} in ''${T_LAST}s"
      done

      for host in "''${HOSTS[@]}"; do
        store_path="''${TOPLEVEL_STORE_PATHS["$host"]}"
        echo "     Copying ➡️ $host"
        nix copy --to "ssh://$host" "$store_path"
        time_next
        echo "      Copied ✅ $host in ''${T_LAST}s"
      done

      for host in "''${HOSTS[@]}"; do
        store_path="''${TOPLEVEL_STORE_PATHS["$host"]}"
        echo "    Applying ⚙️ $host"
        prev_system=$(ssh "$host" -- readlink -e /nix/var/nix/profiles/system)
        ssh "$host" -- /run/current-system/sw/bin/nix-env --profile /nix/var/nix/profiles/system --set "$store_path" \
          || die "Failed to set system profile"
        ssh "$host" -- "$store_path"/bin/switch-to-configuration "$ACTION" \
          || echo "Error while activating new system" >&2
        if [[ -n "$prev_system" ]]; then
          # nvd must be installed on the target system for this to work
          ssh "$host" -- nvd --color always diff "$prev_system" "$store_path" || true
        fi
        time_next
        echo "     Applied ✅ $host in ''${T_LAST}s"
      done
    '';
  };
in
deploy