{ coreutils, jq, nix, nixfmt, syncthing, writeShellScriptBin, }:
writeShellScriptBin "syncthing-set-id" ''
  set -euo pipefail

  PATH="${coreutils}/bin"

  usage() {
    cat <<< '
  usage: syncthing-set-device-id file [name] [id]
  example: syncthing-set-device-id /etc/nixos/modules/syncthing-devices.nix
  example: syncthing-set-device-id /etc/nixos/modules/syncthing-devices.nix lappy
  example: syncthing-set-device-id /etc/nixos/modules/syncthing-devices.nix lappy 1234567-89ABCDE-FGHIJKL-MNOPQRS-TUVWXYZ-0123456-789ABCD-EFGHIJK

  If `name` is not specified, $HOSTNAME is used.
  If `id` is not specified, the ID of the current system instance is used.
  ' >&2
    exit 1
  }

  [[ $# -lt 1 || "$1" == --help ]] && usage
  file=$(realpath "$1")
  name=''${2:-"$HOSTNAME"}
  if (( $# < 3));then
    configDir=$(${nix}/bin/nix --experimental-features nix-command eval --impure --raw --expr '(import <nixos/nixos> {}).config.services.syncthing.configDir')
    id=$(${syncthing}/bin/syncthing -home="$configDir" -device-id)
  else
    id=$3
  fi

  if [[ ! -e "$file" ]];then
    echo "Creating $file" >&2
    echo '{}' > "$file"
  fi

  tmp=$(mktemp "$(dirname "$file")/.$(basename "$file").XXXXXXXXXX")

  # Use `nix-instantiate --eval -E --json | jq -r` instead of `nix eval --raw` until
  # https://github.com/NixOS/nix/issues/2678 is fixed.
  ${nix}/bin/nix-instantiate --eval -E --json \
      --argstr file "$file" \
      --argstr name "$name" \
      --argstr id   "$id" \
    '
      { file, name, id }:
      let pkgs = import <nixpkgs> { };
      in pkgs.lib.generators.toPretty { } (pkgs.lib.recursiveUpdate (import file) {
        services.syncthing.settings.devices."''${name}".id = id;
      })
    ' | ${jq}/bin/jq -r | ${nixfmt}/bin/nixfmt > "$tmp"

  mv "$tmp" "$file"
''