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#+title: systemd
#+date: "2021-05-20 22:37:22 +08:00"
#+date_modified: "2022-09-29 12:21:34 +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.

Among other things, it has the following list of features.

- [[id:f1b21fc8-86a5-47cd-b3d8-da6ac7a34427][systemd timers]] which can replace cron for task scheduling.
- [[id:cd5f0d04-d9bb-44e8-a0f2-630ea58c1e94][systemd services]] along with the usual antics of a service manager such as managing dependencies and commands to run when killed.
- [[id:a602f900-cdcf-4090-9278-d5926d80eedc][systemd transient units]] for quickly creating and scheduling one-off services.
- [[id:14b49597-011c-4da1-b955-bed6059af4a3][systemd unit templates]] is handy for managing units that have common structure, enabling to start them quickly and dynamically.
- [[id:3c67e623-c269-4c9b-9bdf-4ad677d46a35][systemd environment directives]] enable setting environment variables from a systemd-ful session.
- [[id:e4dba4ef-71dd-4d30-9a2c-4ad97223510b][systemd-networkd]] is the network configuration manager in case you want to do [[id:a208dd50-2ebc-404d-b407-3ec2f556535e][Network configuration in Linux]].
- [[id:8505f1f0-f15b-4b04-91fc-12be01913ce6][systemd-boot]] is a bootloader mainly for UEFI-based systems.
- [[id:d83c099a-fc11-4ccc-b265-4de97c85dcbe][systemd-journald]] is the system logging service providing a structured way to manage your logs from different units.
- [[id:7fce893f-418f-42aa-b2b1-59d9f0993406][systemd unit hardening]] can help your services secure.




* systemd at user-level
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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
# See how different the output when run at user- and system-level.
systemctl --user show-units
systemctl show-units

systemctl --user show-environment
systemctl show-environment

systemctl --user start $SERVICE
#+end_src




* Navigating systemd documentation
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Because of the wide scope of the tool, it is best to know where you can find certain information.
Here's an exhaustive list of the locations.

- Most sensible distributions should include the manual pages from the software.
  systemd has a lot of them from knowing [[id:f1b21fc8-86a5-47cd-b3d8-da6ac7a34427][systemd timers]] at =systemd.timers.5=, the unit formats and their locations at =systemd.unit.5=.

- =systemd.directives.7= is an index of configuration directives including unit keys, environment variables, and command line options for systemd-related things.
  Also contains the related manual pages for a deeper references.
  [fn:: How did I pass a year without knowing this?]

- On a similar note, =systemd.index.7= is an alphabetical index of the important keywords found in systemd.





* Extra information

- =systemd.mount= units require the filename to be the mountpoint.
  Though, it has to be converted to what systemd accepts (e.g., =systemd-escape --path $PATH=).

- systemd has a few usual units such as =default.target= which is a target for the current environment of the user.
  It can be get with =systemctl get-default= command.
  More can be seen with =systemd.special.7= from the manual.

- Not all units are equal as systemd treats some units specially.
  You can see what these specific units are and what it does with =systemd.special.7= from the manual.