I have always been a big fan of the batch-scale-level-sharpen plugin, [url=http://registry.gimp.org/node/25350]. Apart from anything else, I use it to scale and recode by raw camera images for more general consumption.
Just today, I went through the labour of making this work from the command line, first in MacOSx, then in Windows.
First install the plugin, by downloading and copying the file to one of the library directories listed in File -> Preference -> Files
I am using 2.8.14 on both. On MacOSx I had to work around some library load path bug, in Windows to struggle with character escaping with CMD.EXE . Never the less, I now have both working & would like to share this.
The error I saw in MacOSx was: