I found this tutorial on gimp.org:
GIMP - GIMP Batch Mode
can someone please tell me how the “simple example” in it is done for GIMP 3?
(let* ((image (car (gimp-file-load RUN-NONINTERACTIVE filename filename)))
(drawable (car (gimp-image-get-active-layer image))))
(plug-in-unsharp-mask RUN-NONINTERACTIVE
image drawable radius amount threshold)
(gimp-file-save RUN-NONINTERACTIVE image drawable filename filename)
(gimp-image-delete image)))