How to convert XCF to PNG using GIMP from the command-line?

Question

As part of my build process I need to convert a number of XCF (GIMP's native format) images into PNG format. I'm sure this should be possible using GIMP's batch mode, but I have forgotten all of the script-fu I used to know.

My input images have multiple layers, so I need the batch mode equivalent of "merge visible layers" followed by "save as PNG". Also note that I can't install anything in ~/.gimp*/scripts/ — I need a self-contained command-line, or a way to install scripts in my source tree.

Note that while this is similar to this question, I have the additional constraint that I need this to be done using GIMP. I tried the current version of ImageMagick and it mangles my test images.

Answer 1

Before jsbueno posted his answer I had also tried asking on the #gimp IRC channel. I was directed to this thread on Gimptalk which contains the following code:

gimp -n -i -b - <<EOF
(let* ( (file's (cadr (file-glob "*.xcf" 1))) (filename "") (image 0) (layer 0) )
  (while (pair? file's) 
    (set! image (car (gimp-file-load RUN-NONINTERACTIVE (car file's) (car file's))))
    (set! layer (car (gimp-image-merge-visible-layers image CLIP-TO-IMAGE)))
    (set! filename (string-append (substring (car file's) 0 (- (string-length (car file's)) 4)) ".png"))
    (gimp-file-save RUN-NONINTERACTIVE image layer filename filename)
    (gimp-image-delete image)
    (set! file's (cdr file's))
    )
  (gimp-quit 0)
  )
EOF

This scriptfu globs for xcf files, and then for each file it loads the file, merges the visible layers, saves the result as a PNG, and "unloads" the image. Finally, it quits GIMP. The glob approach is used to avoid starting up GIMP for each image. It also side-steps the issue of getting parameters from the shell into gimp.

I'm posting this answer just in case someone needs a way to do this without the use of GIMP-Python (perhaps because it isn't installed).