#+title: systemd user services
#+rss_title: systemd user services
#+pubdate: <2021-01-02>

This is a text version of a lighting talk I gave at [[https://www.recurse.com][recurse center]] Feb 2020.

Back when I was first getting into Linux, I had arch installed on my laptop that I was using for schoolwork, and  was trying all the latest ｍｎｍｌ window managers, which meant there was no builtin display for how much battery I had left.

Some googling showed me that you could do the following to get the charge on Linux:

#+begin_src sh
$ cat /sys/class/power_supply/BAT0/capacity
92
#+end_src

That's a pain to type out, so I aliased it to something spam-able like ~a~ and called it a day. But then... surprise shutdowns still happened on occasion. So I just got into the habit of spamming checking all the time due to laptop FOMO. This is not ideal.

Next step was to wrap the above in a script and display it in my panel. Nice! now the battery status is always a glance away. BUT! pixel real estate is precious on my 1366 x 768 resolution, so I would often hide the panel or full-screen something like media, or my text editor. We are back where we started.

** [[#h-9c426974-626d-47cf-bca6-c2b71698b392][Alerts]]
:PROPERTIES:
:CUSTOM_ID: h-9c426974-626d-47cf-bca6-c2b71698b392
:END:

With something like [[https://github.com/dunst-project/dunst][dunst]], I can send notifications that appear only when I'm concerned with them. This birthed a script that would only bother me if my battery was below a certain level, while polling periodically. I decoupled that last part into a script named [[https://github.com/neeasade/dotfiles/blob/master/wm/.wm/scripts/services/periodically][~periodically~]], so ended up with a ~battery_alert~ daemon-like script that looked like ~periodically 60 check_battery~.

Making this call in my ~.xinitrc~ worked out OK, but I'm sometimes at a desktop using the same set of dotfiles, and there's no battery to check there. About this point I realized I had stuffed a lot in my startup, such as ~mpd~, ~picom~, ~dunst~, you name it. I had heard of systemd user services and decided to look into them.


** [[#h-bedee5e8-3ef1-4038-9bc8-33640146db06][The big D]]
:PROPERTIES:
:CUSTOM_ID: h-bedee5e8-3ef1-4038-9bc8-33640146db06
:END:

Systemd user services are regular systemd service files that live in ~$HOME/.config/systemd/user~. A service file matching the above problem might look like this:

~battery_alert.service~:
#+begin_src conf
[Unit]
Description=Notify when the battery is low

[Service]
ExecStart=/path/to/bin/periodically 60 check_battery
ExecStop=/path/to/bin/kill -int $MAINPID

Environment=PATH=/paths/that/contain/check_battery:...:....
Environment=DISPLAY=":0"
Environment=XAUTHORITY="/home/neeasade/.Xauthority"

[Install]
WantedBy=default.target
#+end_src

Some things to note:

- The full path to the ~ExecStart~ and ~ExecStop~ executable
- The inclusion of ~DISPLAY~ and ~XAUTHORITY~ -- this is so the ~notify-send~ reaches dunst
- The setting of ~PATH~ such that "the usual suspects" are available within my daemon script

With the above I get the systemd goodness but for managing my local stuff, just passing ~--user~:

#+begin_src shell
# start, stop
systemctl --user start battery_alert
systemctl --user stop battery_alert

# logs (can also tail theme)
systemctl --user status battery_alert

# automatic startup
systemctl --user enable battery_alert
#+end_src

So the next step in this systemd journey was to integrate [[https://github.com/neeasade/dotfiles/blob/cd6a4cd7c8207b7300bf0bf75f87aee8a7e4fbc1/wm/.wm/scripts/theming/ltheme#L379-L465][service generation]] into my dotfiles setup, generating a number of service files that contain full paths to binaries with intended arguments for a range of daemons.

** [[#h-42fac00e-07ec-4d8d-849f-eaf1a86bb935][The big.. T?]]
:PROPERTIES:
:CUSTOM_ID: h-42fac00e-07ec-4d8d-849f-eaf1a86bb935
:END:

Those of you in the know will see something very wrong with the above. Systemd has a "when to run" concept: [[https://www.freedesktop.org/software/systemd/man/systemd.timer.html][timers]]! Using timers files you can do things like:

- run on boot
- run on login
- run once per day/once per time interval
- run *periodically*

Let's take a look at what that last one looks like. You create a file in the same directory with the same name, but extension timer:

~battery_alert.timer~:
#+begin_src conf
[Unit]
Description=Run check_battery every 60 seconds

[Timer]

# on boot
# OnBootSec=0min

# on service manager activation
OnStartupSec=0min

OnUnitActiveSec=60s

[Install]
WantedBy=timers.target
#+end_src

See the reference for time-span syntax [[https://www.freedesktop.org/software/systemd/man/systemd.time.html#][here]]

And then we would change the ~ExecStart~ in our ~battery_alert.service~ file to:

#+begin_src conf
ExecStart=/path/to/check_battery
#+end_src

And bam! we have done the thing.