Hey there, I’ve helped tons of users work through this exact error when trying to run user-level systemd services, so let’s break down the most reliable fixes to get your service up and running as a regular user.

First: What’s Causing This? #

This error almost always traces back to one of two core issues:

• The systemd user instance for your regular account isn’t running (or shuts down when you log out)
• Critical environment variables like DBUS_SESSION_BUS_ADDRESS or XDG_RUNTIME_DIR aren’t properly initialized for your user session

Step 1: Enable User Linger (The Most Critical Fix) #

By default, systemd stops a user’s service instance as soon as they log out. To keep it running even when you’re not logged in (and ensure the D-Bus connection stays intact):

1. Use sudo or switch to root to run:

   sudo loginctl enable-linger your-regular-username
   

2. Verify it worked with:

   loginctl show-user your-regular-username | grep Linger
   

   You should see Linger=yes in the output.

Step 2: Use a Proper User Session #

If you’re switching users or logging in via SSH, you need to load the full user environment:

• Instead of su your-regular-username, use:

  su - your-regular-username
  

  The - flag ensures all your user’s shell profiles and required environment variables (including those for D-Bus) are loaded.
• For SSH, avoid running single commands directly (like ssh user@host systemctl --user start service). Instead, log into an interactive shell first, or pass the command with a login shell:

  ssh user@host -t "bash -l -c 'systemctl --user start your-service'"
  

Step 3: Manually Set Environment Variables (If Needed) #

If the above steps don’t resolve the issue, you can manually define the D-Bus variables for your session:

1. As your regular user, run:

   export XDG_RUNTIME_DIR=/run/user/$(id -u)
   export DBUS_SESSION_BUS_ADDRESS=unix:path=${XDG_RUNTIME_DIR}/bus
   

2. Test if the connection works with:

   systemctl --user list-units
   

   If this outputs your user’s services without errors, you’re ready to start your target service.

Step 4: Install Missing D-Bus Packages (For Minimal Systems) #

On stripped-down systems (like containers or minimal OS installs), you might be missing the user-level D-Bus package:

• For Debian/Ubuntu-based systems:

  sudo apt install dbus-user-session
  

• For RHEL/CentOS/Fedora:

  sudo dnf install dbus
  

Reboot your system after installing to ensure the D-Bus session initializes properly.

Final Check #

Once you’ve applied the fixes, test starting your service again:

systemctl --user start your-service-name

You can confirm it’s running with:

systemctl --user status your-service-name

内容的提问来源于stack exchange，提问作者Joachim