bindfs

# installing
apt-get install bindfs

# link a folder to nextcloud
bindfs -u www-data -g nogroup -p 0000,u=rD /mnt/plex/ /var/www/nextcloud/data/files/PLEX/

# Linking for Resilio Sync / BTSync / SyncThing
src="/mnt/data/foo"
dst="/home/btsync/$(basename "${src}")"
mkdir -p "${dst}"
bindfs -u btsync -g btsync "${src}" "${dst}"