Environment variable generators for systemd services

On my GNU/Linux computer, I do not use a login manager. I log in from the tty. And if the tty that I am logging in from is tty1, I start the Hyprland wayland compositor.

The following is at the very end of my ~/.profile file.

if [ $(tty) = '/dev/tty1' ]
then
    exec dbus-launch --autolaunch=$(cat /var/lib/dbus/machine-id) Hyprland
fi

I also setup all the environment variables, early in the ~/.profile file. Most of these environment variables like PATH, XDG_CONFIG_HOME, XDG_CACHE_HOME etc. affect the behavior of other programs. While there are also a handful, that I use in my personal shell scripts.

So far, this setup was working very well. The environment variables were setup, then the wayland session was started. And as a result, all the applications that I launched in that session inherited the environment variables.

But the problem arose when I started using systemd user services. I noticed that when emacs was started by systemd as a user service, it no longer had those environment variables set up.

Section 2.1 of the ArchWiki systemd/User page says:

Units started by user instance of systemd do not inherit any of the environment variables set in places like .bashrc etc

Now what?

The same ArchWiki page also lists the ways in which environment variables can be set for the services. The first few solutions suggest to put .conf files containing lines in the form of NAME=VALUE, into certain directories. This would have worked for static key-value pairs. But in my case, I needed to set certain variables (like PATH) dynamically.

Hence, I went for the systemd.environment-generator(7) solution.

A environment-generator is an executable, that systemd will invoke very early on, before starting any service. This environment-generator executable is supposed to print lines in the format of NAME=VALUE to stdout. systemd will then set NAME environment variable to VALUE and when it starts a service, these environment variables will be visible to them.