In addition to UEFI Shell scripts, Python, and Lua, you can also use Ruby to write code for UEFI.
Mruby is the Ruby compiler that was ported to UEFI. “mruby is the lightweight implementation of the Ruby language complying to (part of) the ISO standard. Its syntax is Ruby 1.9 compatible.”
Mruby on EFI Shell is a mruby port to the UEFI Shell, ported by Masamitsu Murase. With mruby.efi, you can call UEFI BootTime and RunTime Services, and access UEFI data structures. For some nice examples, look at the home page of the project.
To build mruby for EFI Shell, look at the readme on the sources, you need to create a new subdirectory in the EDK-II AppPkg for it. Once built, you need to copy mruby.efi onto your UEFI System Partition (ESP) so you can access it via the UEFI Shell. From the UEFI Shell, sample usage is:
This has been around for about 3 years, but I only noticed it a few weeks ago.