#!/bin/bash
# set the default value
XDG_DATA_DIRS="${XDG_DATA_DIRS:-/usr/local/share/:/usr/share}"
 
# add a directory if it exists
if [[ -d /opt/foo/share ]]; then
   XDG_DATA_DIRS=/opt/foo/share:${XDG_DATA_DIRS}
fi
 
# write our output
echo XDG_DATA_DIRS=$XDG_DATA_DIRS