How to start a systemd service after mount command

Question

I am working on a lab for my linux class and the teacher is teaching us about Systemd.

How do I create a service that starts a script when a specific device is mounted using this command: mount /dev/xvdc1 /mnt/backup

What I have tried

1. Using a Timer I checked if /mnt/backup was a mount point. (teacher would not accept it. It must be strictly event driven)
2. Creating a .mount unit and making the .service unit require the .mount unit. This approach did not start the script when executing the command: mount /dev/xvdc1 /mnt/backup

I think I may be approaching this problem the wrong way. This is the lab problem.

"Create a unit file that copies all the files from /mnt/backup using rsync as soon as /dev/xvdc1 is mounted."

What I know:

1. The device that will be mounted will always be /dev/xvdc1
2. The mount point of the device will always be /mnt/backup
3. The Service must be started based on /dev/xvdc1 being mounted

Here is my .service unit file

[Unit]
Description=Starts a backup for /dev/xvdc1 when mounted using the mount command.

[Service]
Type=simple
ExecStart=/root/backupscript.sh
ExecStop=/bin/kill -HUP $MAINPID

[Install]
WantedBy=multi-user.target

Answer 1

Systemd Unit Start Conditions

With ConditionPathExists= a file existence condition is checked before a unit is started. If the specified absolute path name does not exist, the condition will fail. If the absolute path name passed to ConditionPathExists= is prefixed with an exclamation mark ("!"), the test is negated, and the unit is only started if the path does not exist.

ConditionPathIsMountPoint= is similar to ConditionPathExists= but verifies whether a certain path exists and is a mount point.

Seems like the easiest method is to include

ConditionPathIsMountPoint=/mnt/backup

In your [Unit] section. Though this doesn't check which device is mounted there, though in the case of creating a backup the user may not be picky.

You'd probably also need Restart=on-failure to continue trying to start the service.

---

A more precise method would be to write a udev rule that starts the service when a particular device is mounted.

As a reference

Start by finding your device in lsusb. Note the ID (eg 0a81:0101)

Create a new udev rules file in /etc/udev/rules.d/ via sudoedit /etc/udev/rules.d/100-mount-videos.rulesand plonk a new rule in there like this:

ACTION=="add", ATTRS{idVendor}=="0a81", ATTRS{idProduct}=="0101", RUN+="/home/your_username/bin/mount_videos.sh"

You could just replace the RUN+= with RUN+=/usr/bin/systemctl start backup.service or whatever you called your service