Completely untested Nix builder for icon theme with only "image-missing.png"

only-missing.nix

{ stdenv
, pkgs
, gnome3
, hicolor-icon-theme
}:

stdenv.mkDerivation {
  pname = "only-missing";
  version = "0.0.1";

  propagatedBuildInputs = [ hicolor-icon-theme ];
  buildInputs = [ gnome3.adwaita-icon-theme ];

  builder = pkgs.writeScript "only-missing-builder.sh" ''
    . ${stdenv}/setup

    mkdir -p $out/share/icons/only-missing/

    declare -a directories
    for f in "${gnome3.adwaita-icon-theme}/share/icons/Adwaita"/*; do
      if [[ ! -d "$f" ]]; then
        continue
      fi

      size="''${f##*/}"

      if [[ -f "$f/legacy/image-missing.png" ]]; then
        mkdir -p $out/share/icons/only-missing/$size/legacy
        cp -v "$f/legacy/image-missing.png" $out/share/icons/only-missing/$size/legacy/
        directories+=("$size/legacy")
      fi

      if [[ -f "$f/status/image-missing.png" ]]; then
        mkdir -p $out/share/icons/only-missing/$size/status
        cp -v "$f/status/image-missing.png" $out/share/icons/only-missing/$size/status/
        directories+=("$size/status")
      fi
    done

    theme=$out/share/icons/only-missing/index.theme

    cat <<EOF > $theme
    [Icon Theme]
    Name=only-missing
    Comment=Fallback icon theme with image-missing
    Hidden=true
    EOF

    ( export IFS="," ; echo "Directories=''${directories[*]}" ) >> $theme

    for dir in "''${directories[@]}"; do
      size="''${dir%%x*}"
      name="''${dir##*/}"

      cat <<EOF >> $theme

    [$dir]
    Size=$size
    Context=''${name^}
    Type=Fixed
    EOF
    done
  '';

}