Seeing as the favicon standard dates back to 1999, it is difficult to reason why they are still a pain to generate. The following outlines a workflow to generate a site's favicon using Sketch.

Setup

Sketch ships with a little known CLI called sketchtool. Installation is straight forward:

/Applications/Sketch.app/Contents/Resources/sketchtool/install.sh
which sketchtool # /usr/local/bin/sketchtool

Next imagemagick is needed. If it isn't installed it is easiest to setup through brew:

brew install imagemagick
which convert # /usr/local/bin/convert

With both sketchtool and imagemagick installed grab generate a sketch file with a favicon. Ensure the file includes an artcboard with exports for your preferred favicon sizes (e.g. 16 by 16, 32 by 32, etc).

Task

Now that the Sketch file is setup add the following Rake task to the project as lib/tasks/favicon.rake:

namespace :favicon do
  desc "generate a favicon"
  task generate: :environment do
    system <<~SYSTEM
    sketchtool export artboards #{Rails.root}/doc/favicon.sketch --output=#{Rails.root}/tmp
    convert #{Rails.root}/tmp/favicon-*.png #{Rails.root}/public/favicon.ico
    rm #{Rails.root}/tmp/favicon-*
    SYSTEM
  end
end

Enjoy

That is it! Anytime the doc/favicon.sketch file is modified running rake favicon:generate generates public/favicon.ico. This technique also works surprisingly well for other assets.

Advanced

To generate a favicon Sketch file from scratch go 'File' then 'New'. Insert an artboard using 'Insert' then 'Artboard'. The artboard can use any dimensions that are square (if in doubt go with 64×64). Rename the artboard to 'favicon' then make it exportable. Generate export sizes (if in doubt go with 16w, 24w, 32w, 48w, and 64w) and apply a corresponding 'Prefix/Suffix' (i.e. for 16w use ...-16) as PNG. The Sketch file is ready for use! Save the document to doc/favicon.sketch in the project.

If the favicon is changed often it is a good idea to modify the task to convert to app/assets/images/favicon.ico. Then adjust the app/views/layouts/application.html (or other layout) to contain favicon_link_tag 'favicon.ico', sizes: '16x16 24x24 32x32 48x48 64x64' (see docs). This uses the asset pipeline and ensures browsers detect changes to the favicon.