Path

The final step is to set up the systemd units that will monitor and trigger the execution of powerchange.sh.

You use path units to monitor files and directories. As you will be using upowerlog.sh to update the upower.log file, upower.log is what you want to monitor. The path unit shown in Listing 4 does that.

01 [Unit]
02 Description= monitoring upower log
03 Wants= upowerlog.service
04
05 [Path]
06 PathModified= /home/[username]/.local/share/upower.log

We already talked about the Description directive when explaining upowerlog.service and the PathModified directive is self-explanatory: It points to a file or a directory and is triggered if said file or directory is modified.

There is another new directive in powerchange.path worth looking at in a little more depth, though. Line 3 shows a Wants directive that points to the first unit you created, upowerlog.service. This ensures that if upowerlog.service is not started when you run powerchange.path, systemd will get it started. This means you don't have to run upowerlog.service yourself; systemd will do it for you the moment it parses the Wants= upowerlog.service line.

You will notice that there is no call to powerchange.sh, the script we created in Listing 3. That's because path units do not execute scripts themselves. Instead they rely on a companion service script (usually with the same name, but with the .service extension) to do that for them. After all, that is what service units are for: starting scripts or programs.

powerchange.service looks like what is shown in Listing 5. Again, this service is self-explanatory and all it does is run the powerchange.sh script from Listing 3.

01 [Service]
02 Type= simple
03 ExecStart= /home/[username]/.local/bin/powerchange.sh

Save both powerchange.path and powerchange.service to /home/[username]/.config/systemd/user/, and you can now run powerchange.path with

systemctl --user start powerchange.path

and check that everything works by unplugging your laptop. The notification shown in Figure 6 should pop up, warning you that your machine has been disconnected.

Walkthrough

To summarize what's going on:

1. 1 When you start powerchange.path, systemd also starts upowerlog.service thanks to the Wants directive in powerchange.path.
2. 2 upowerlog.service runs upowerlog.sh, which starts dumping the output of upower -m into /home/[username]/.local/share/upower.log.
3. 3 Each time /home/[username]/.local/share/upower.log changes, powerchange.path calls powerchange.service.
4. 4 powerchange.service runs the powerchange.sh script that checks to see if the line power source is connected. If it is not, it sends a notification to the desktop.

Conclusions

I hope this has helped illustrate how you can leverage a wide variety of seemingly unrelated tools to create one single project.

In any case, systemd units, the UPower system, notify-send (and other command-line-to-desktop utilities), and Bash's daisy-chaining of terminal commands are all worth exploring further. They provide you with a rich set of tools you can use to complete all kinds of projects.