man/50-xdg-data-dirs.sh

#!/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