> Fair enough, but why is there no standard shell or alternative interpreter?

There actually exists the UEFI Shell, but not every UEFI implementation has it built in. For example the UEFI implementation that Intel provides for the Minnowboard Max (now EOL) and Minnowboard Turbot does provide an UEFI Shell.

If the producer of the mainboard/laptop has not built in the UEFI Shell, there still exist options to start an UEFI Shell binary as an ordinary UEFI application (just as a bootloader etc. is also just an UEFI application):

> https://superuser.com/a/1057594/62591

In principle it should even be possible to write an alternative shell for UEFI and start it this way, but I am not aware that someone has written one.