Dear all,
I have used heif-enc to convert my jpg files.  Very few of them just turn out totally black.  Therefore, gimp becomes the alternative one.  The OS is ubuntu 18.04 and gimp 2.10 is installed from the ubuntu software as default.
The small shell program follows:
==============================================
#!/bin/bash
# quality level
ql=0; while [[ ! $ql =~ [[:digit:]] || $ql -lt 50 || $ql -gt 100 ]] ; \
   do  read -e -p "Enter quality 50-100 : " ql; ql=${ql/.*} ;done
for f in $(find . -iname "*.jpg" -type f 2> /dev/null)
  do
infile=${f%.*}.jpg
outfile=${f%.*}.heic
gscript="(let* ((image (car (file-jpeg-load 1 \"$infile\" \"$infile\")))
                (drawable (car (gimp-image-active-drawable image)))
               )
               (file-heif-save 1 image drawable \"$outfile\" \"$outfile\" $ql 0)
         )"
#           ; 50 quality (int32 0 <= x <= 100)
#           ;  0 use lossless compression (0 = lossy, 1 = lossless)
echo ""
echo $f
echo ""
gimp -c -i -d -b "$gscript" -b "(gimp-quit 0)"
done
==================================================
I am happy it generates the heic files.  However, the warning message is quite annoying.  What can I do to avoid it?   The message shows:
=================================================
gimp_check_updates_callback: loading of https://www.gimp.org/gimp_versions.json failed: Operation not supported
batch command executed successfully
/snap/gimp/347/usr/bin/gimp: GEGL-WARNING: (../gegl/buffer/gegl-tile-handler-cache.c:1076):gegl_tile_cache_destroy: runtime check failed: (g_queue_is_empty (&cache_queue))
EEEEeEeek! 2 GeglBuffers leaked
To debug GeglBuffer leaks, set the environment variable GEGL_DEBUG to "buffer-alloc"
=================================================
Thanks,
Chuck