Creating a simple laptop notification
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.
Listing 4
powerchange.path
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.
Listing 5
powerchange.service
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 When you start
powerchange.path
, systemd also startsupowerlog.service
thanks to theWants
directive inpowerchange.path
. - 2
upowerlog.service
runsupowerlog.sh
, which starts dumping the output ofupower -m
into/home/[username]/.local/share/upower.log
. - 3 Each time
/home/[username]/.local/share/upower.log
changes,powerchange.path
callspowerchange.service
. - 4
powerchange.service
runs thepowerchange.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.
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