Memo to self: I have a VPS with a legacy Ubuntu 20.04, and when creating a user to run a user-based service, trying to use systemctl fails with the message:

$ systemctl --user status
Failed to connect to bus: No such file or directory

To solve this, multiple changes were necessary:

Fixing the systemd problem

The service is supposed to run as the user theservice, from the directory /home/theservice/therepo, and is to be controlled by a systemd instance for this user. There was no such instance running.

Missing packages

Several required packages were not installed (server image, minimal packages installed):

# apt install dbus-user-session libpam-systemd libpam-cgfs

loginctl config not correct

Loginctl needs to be told what to do when the user is not logged in:

# loginctl enable-linger theservice

Missing environment variables

Two environment variables were not defined properly:

$ cat ~/.bashrc 
# ~/.bashrc: executed by bash(1) for non-login shells.
# see /usr/share/doc/bash/examples/startup-files (in the package bash-doc)
# for examples

# THIS INSERTED vvv
export XDG_RUNTIME_DIR=/run/user/$(id -u)
export DBUS_SESSION_BUS_ADDRESS="unix:path=${XDG_RUNTIME_DIR}/bus"
# THIS INSERTED ^^^

# If not running interactively, don't do anything
case $- in
    *i*) ;;
      *) return;;
esac
...

Note: As you can see in the case statement and the comment before, the remainder of the .bashrc is only run for interactive shells. The variable definitions must appear before the case statement, as shown.

user@.service not enabled and started

The user-based systemd component was not enabled and started.

# systemctl enable user@.service
# systemctl start user@1011.service    # the userid of the user I needed

Defining the service

The service we want to run is a Python program in a virtual environment, logging to stdout and stderr. It needs to be started by systemd as the service user.

We check out the repo and create virtual environment:

# sudo -Hi theserviceuser
$ git checkout git@github.com:theuser/therepo.git
...
$ python3 -m venv venv
$ source venv/bin/activate
$ pip install -r requirements.txt
...

This will check out the source, create a venv and then activate it and install the requirements.

We can now create a service. As the service user, run systemctl --user edit --full theservice.service and install

# /home/theserviceuser/.config/systemd/user/theservice.service
[Unit]
Description=The Service
After=syslog.target network.target

[Service]
Type=simple
WorkingDirectory=/home/theserviceuser/therepo
ExecStart=/home/theserviceuser/therepo/venv/bin/python3 /home/theserviceuser/therepo/main.py
Restart=on-abort
EnvironmentFile=/home/theserviceuser/therepo/.env

[Install]
WantedBy=multi-user.target

By using the Python instance from the venv, we will automatically use stuff from the venv, no activation required.

We can now update all this with

$ systemctl --user stop theservice.service
$ git pull --rebase
$ systemctl --user start theservice.service

or a script that does the same.