[Unit]
Description=Backup the content of /mnt/test
Documentation=https://github.com/nmasse-itix/Systemd-Examples
Before=mnt-test.mount
RequiresMountsFor=/mnt/backup

[Service]
Type=oneshot
UMask=077
ExecStart=tar -cf /mnt/backup/mnt-test.tar -C /mnt/test .