wiki/structured/linux.systemd.org

#+title: Exploring systemd features
#+date: "2021-05-20 22:37:22 +08:00"
#+date_modified: "2021-06-09 10:29:31 +08:00"
#+language: en
#+property: header-args  :eval no


systemd is a big tool for a big system.
Let's explore some of them from a perspective of a wannabe power user.




* systemd at user-level

systemd has the ability to run at user-level empowering the user to manage their own system with their own settings.
It immensely helps separating user-specific settings from the system-wide settings.

systemd looks for the units from certain paths.
You can look for them from the =systemd.unit.5= manual page.

To run systemd as a user instance, simply add a =--user= flag beforehand for =systemctl= and other systemd binaries, if applicable.

#+begin_src shell  :results none
systemctl --user show-units
systemctl --user show-environment
systemctl --user start $SERVICE
#+end_src




* Timers as cron replacement

You can schedule tasks with timers.
If systemd is compiled with the feature, it makes cron unnecessary.

systemd has different ways to denote time.

- Timespans denote the duration — e.g., =100 seconds=, =5M 3w=.
- Timestamps refer to a specific point in time — e.g., =2021-04-02=, =today=, =now=.
- Calendar events can refer to more than one point of time — e.g., =*-*-4/2=, =Sun,Wed,Fri *-1/2-1/8=.

Here's an example of setting a timer for an example backup service.
The following timer unit sets it to execute every day at 18:00.

#+begin_src ini
[Unit]
Description=A deduplicated backup from my computer
Documentation=man:borg(1) https://borgbackup.readthedocs.io/

[Timer]
Unit=borg-backup.service
OnCalendar=*-*-* 18:00:00
Persistent=true

[Install]
WantedBy=graphical.target
#+end_src

This will trigger =borg-backup.service= from the load path.
But you can omit it if you named the timer unit file similarly (e.g., =borg-backup.timer= with =borg-backup.service=).

You can find more information about it from the =systemd.time.5= manual page.
Furthermore, systemd has a testing tool for time with ~systemd-analyze {timespan,timestamp,calendar}~.




* Unit templates

You can create unit templates which is useful for simple services that only requires an argument.
Rather than creating individual simple service files, let systemd handle it.

For example, you may want to spawn a service for Borgmatic with multiple repos.
If you don't know templates, the dumb way to serve multiple repos is to create individual unit files for each.
If you want to schedule them, you also have to create a timer unit for each.

The more efficient solution is to use templates.
To make a unit template, there are only a handful of requirements:

- Addition of =%i= to represent the template value.
- The unit file name has to end with =@= (e.g., =unit-name@.service=, =unit-name@.timer=).

This could be compressed into a template for a service unit.
The following code shows how to create one.

#+begin_src ini
[Unit]
Description=Periodic safety backup for %i
Documentation=man:borg(1) https://www.borgbackup.org/

[Service]
Type=simple
ExecStart=borgmatic --config %i --verbose

[Install]
WantedBy=default.target
#+end_src

To use the service, you have to give it a value — e.g., ~systemctl --user start borg-backup@test.yaml.service~.

That's all good but what about scheduling them?
What if you want to create an archive every hour starting at 08:00?
You can just create a templated timer unit.

#+begin_src ini
[Unit]
Description=Periodic safety backup for %i
Documentation=man:borg(1) https://www.borgbackup.org/

[Timer]
Unit=borg-backup@%i.service
Calendar=08/1:00:00
Persistent=true

[Install]
WantedBy=default.target
#+end_src




* Transient units

You can create units on-the-go with =systemd-run=.
It generates transient unit files.
Though, this is oriented around service units, making it useful for one-time configurations and task scheduling.

Like most systemd-related binaries, this can configure in system- and user-level.

#+begin_src shell
# This will create a user-level service file with the given command as the task.
systemd-run --user borgmatic --config emergency-config.yaml --verbose

# Create a transient timer for the service.
systemd-run --user borg-backup@external-drive.service --on-calendar=12:00
#+end_src