Following on from my post of how to batch convert Garmin Fit files to GPX using Babel GPS in Windows, thought I'd follow it up with the equivalent Linux command. Obvs you need gpsbabel installed but run this command in the directory you are storing the files and you are away

find . -maxdepth 1 -iname "*.fit" -exec gpsbabel -rt -i garmin_fit -f ./{} -x track,course,speed -o gpx -F ./{}.gpx ';' -exec bash -c 'mv "$1.gpx" "${1%.fit}".gpx' - '{}' ';'