Reducing image size with GIMP in batch mode

 First we define a function which contains all the needed procedures. and save the script in

~/.gimp-2.8/scripts

with .scm extension.

In 2.10, the script directory is ~/.config/GIMP/2.10/scripts

(define (batch-reduce-img-size pattern reduceby)
  (let* ((filelist (cadr (file-glob pattern 1))))
    (while (not (null? filelist))
           (let* ((filename (car filelist))
                  (image (car (gimp-file-load RUN-NONINTERACTIVE filename filename)))
                  (imageWidth (car (gimp-image-width image)))
		   (imageHeight (car (gimp-image-height image)))
		   (newWidth (* imageWidth reduceby))
		   (newHeight (* imageHeight reduceby))
                  (drawable (car (gimp-image-get-active-layer image))))
				   (gimp-image-scale image newWidth newHeight)
                                  (gimp-file-save 1 image drawable filename filename)
                                  
(gimp-image-delete image))
           (set! filelist (cdr filelist)))))

run the script as 

gimp -i -b '(batch-reduce-img-size "*.JPG" 0.5)' -b '(gimp-quit 0)

IMPORTANT: The images will get over written