Simple Systemd service for monitor auto-configuration
June 25, 2017
I like to experiment with my laptop testing different combinations of operating systems and desktop managers but it has been a while my systems are quite stable running Xubuntu + i3. Despite I like the minimalistic simplicity of i3, sometimes it requires some kind of hacks to do stuff usually straightforward in other more user-friendly systems. This post is about a background utility auto-configuring monitor devices connected to my Xubuntu platform.
To configure the monitors I wrote a simple bash script but I took the chance to integrate it in the new Systemd Ubuntu service manager.
Systemd (and its main command systemctl) works with units. A unit any of the system object of interest:
- a service
- a device
- a mount
- a target
All the units share some common properties and methods and each of them can have specific methods. Each unit is described by a text file named:
Targets are special unit files which represent system checkpoints, something similar to the old-fashioned run_levels. Target units are used to group the other units. Given a speicific target unit, a generic unit can specify that:
- it is part of a it (WantedBy)
- it needs it as a pre-condition (Wants)
On start-up the computer will go through all the pre-requisite cascade and reach the default target (which is generally called multi_user.target on a server and graphical.target on a desktop). In any case, Systemd developers are smart enough to expose a default.target which is a symlink to the current default target unit.
System or user services
Systemd differentiates between system services and user services; the latters have less privilegies (hence are safer) and can be enabled by a subset of the entire system users. Technically, the only difference between the two kind of services is the installation folder.
On *buntu, system services are located in /etc/systemd/system while the user services in /etc/systemd/user.
I chose the user option and I bundled my script using an on-the-fly buildeb script.
Note that user services must be enabled manually by the users with:
$> systemctl --user enable <service_name>.service
Our little service should start with the reaching of the default target and we can add a dependency on the multi-user target as it is a user service so it is run only by user sessions. Here it is the service unit file:
[Unit] Description=XWizard automatically configure available monitors using xrandr Wants=multi-user.target [Service] TimeoutStartSec=1 ExecStart=/usr/bin/xwizard [Install] WantedBy=default.target
To install it on a *buntu system, just type:
$> git clone https://github.com/lucabaldesi/XWizard $> ./xwizard.buildeb #> dpkg -i XWizard.deb $> systemctl --user enable xwizard.service