Systemd Linger Feature Clarified for Debian Users

Systemd Process Termination on Logout

Systemd includes a feature that can terminate user processes when a user logs out. This functionality is controlled by the `KillUserProcesses` option within the `/etc/systemd/logind.conf` configuration file for `systemd-logind`. In recent Debian releases, this option is disabled by default, allowing user processes to continue running after logout.

The Role of the Linger Feature

Contrary to a common belief, the "linger" feature is not necessary for applications like screen, tmux, or nohup to persist after a user logs out in Debian's default setup. While some specific configurations might require it, the primary effect of "linger" on a default Debian system is to allow programs to start in the user's session even when the user is not logged in.

Managing User Process Killing

If `KillUserProcesses` is enabled, systemd can be configured to kill processes for specific users or exclude others, such as the root user. A systemd unit can also be used to start applications like screen, preventing them from being terminated upon logout even when `KillUserProcesses` is active. This feature can be useful for managing misbehaving desktop environments by ensuring all user processes are cleared on logout.

Controlling Linger for User Accounts

Users can enable or disable the "linger" feature for their own accounts using the commands `loginctl enable-linger` and `loginctl disable-linger`, provided `polkit` is installed and configured typically. Root users can manage linger for other accounts with `loginctl enable-linger $ACCOUNT` and `loginctl disable-linger $ACCOUNT`. There is no documented command to list accounts with linger enabled; checking `ls /var/lib/systemd/linger` is the current method.