wiki/notebook/linux.systemd.org

Exploring systemd features

• systemd at user-level
• Unit templates
• Bootloader configuration
• Network manager configuration
• DNS server configuration
• Log management
• Environment directive

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.

• systemd timers which can replace cron for task scheduling.
• systemd services along with the usual antics of a service manager such as managing dependencies and commands to run when killed.
• systemd transient units for quickly creating and scheduling one-off services.

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.

# 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

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.

[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

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.

[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

Bootloader configuration

systemd also comes with a bootloader aptly named systemd-boot though it only supports UEFI-based firmware. Just like GRUB, they can be configured through plain-text files.

For detailed information about the bootloader, see the manual page systemd-boot.7.

With a complete installation, the bootloader config folder may look like the following list.

/boot/
`-- loader
    |-- entries             # (ref:loader-entries)
    |   `-- arch.conf
    |-- loader.conf         # (ref:loader-conf)
    `-- random-seed

• loader/entries/ is a directory containing all of the entries available to be booted.
• loader.conf contains the loader configuration.

Most Linux distros with systemd installed should have a sample config file somewhere. ¹ As an example, we'll show what those look like.

loader.conf is the configuration for the boot loader including the timeout seconds among others. Here is a sample of a bootloader configuration.

default arch
timeout 4

In this config, this simply makes the arch loader entry to be default when no actions has occurred. It will start loading it automatically after a timeout of 4 seconds.

The arch loader entry can be found at ${ESP}/loader/entries/arch.conf. The following code block shows what a loader entry looks like.

title   Arch Linux
linux   /vmlinuz-linux
initrd  /initramfs-linux.img
options root="PARTUUID=${PARTUUID}"

You can customize and create extra entries for the same installation. This is what roam:NixOS does with its package generations, letting the user to boot to a specific point in time from the boot loader. Very useful for emergency boots in case the current generation breaks for whatever reason.

For complete details of the configuration file, you can see loader.conf.5 manual page.

Network manager configuration

With a systemd-ful environment, you can run the network daemon (i.e., systemd-networkd). Once enabled, you can run networkctl to list all of the network devices. ²

networkctl

IDX LINK   TYPE     OPERATIONAL SETUP
  1 lo     loopback carrier     unmanaged
  2 enp1s0 ether    routable    configured
  3 wlan0  wlan     routable    configured

3 links listed.

To configure network manager, you can create a network file in one of systemd unit file paths in the system. Each of the device will be assigned an IP address. You can either assign an IP address or dynamically assign them in some way. One of the common ways to do dynamic IP addresses is installing a DHCP server (which is another thing to be configured). Here's an example of configuring any wireless devices and assigning a dynamic IP addresses with DHCP.

[Match]
Type=wlan

[Network]
DHCP=yes
IPv6PrivacyExtensions=yes

[DHCPv4]
RouteMetric=1024

[DHCPv6]
RouteMetric=1024

DNS server configuration

While the network manager is enabled, you can access the internet. But only with raw IP addresses (e.g., 1.1.1.1 from Cloudflare, 93.174.95.27 for Library Genesis). ³[dog]] or the DNS library from NodeJS.]

Accessing the domain names as you would browse the web normally is an additional layer of the web. To access a domain name, you need a DNS client that can resolve them. While there are plenty of DNS resolvers, systemd has a component systemd-resolved which you can control with resolvectl binary.

systemd-resolved takes a configuration from /etc/resolve.conf which most third-party programs also relies on.

Log management

systemd has a journal service storing logs from units. It provides a consistent and structured way how to check the logs.

It uses journalctl as the command for managing logs. See Command line: journalctl for more details.

journalctl --user-unit backup.service --follow --boot

Environment directive

systemd enables setting the environment through environment directives.

Just like how systemd at user-level, you can set it at user-level. Useful for setting specific environment only for that user.

It needs a *.conf file in one of the load paths (seen from the environment.d.5 manual page). Keep in mind it does not use the shell and instead makes use of shell-like syntax.

The following code block is an example of setting Nix-related environment variables to enable desktop integrations.

# Enable desktop integration with Nix-installed applications.
NIX_PATH=$HOME/.nix-defexpr/channels${NIX_PATH:+:}$NIX_PATH
PATH=${PATH:+$PATH:}$HOME/.nix-profile/bin
XDG_DATA_DIRS=${XDG_DATA_DIRS:+$XDG_DATA_DIRS:}$HOME/.nix-profile/share/

---

¹

In case of Arch Linux, it has an example file at /usr/share/systemd/bootctl/.

²

You can also run ip address for it.

³

You can find the IP addresses with DNS clients such as [[https://github.com/ogham/dog