systemd-logind is a system service that manages user logins. It is responsible for:
Keeping track of users and sessions, their processes and their idle state. This is implemented by allocating a systemd slice unit for each user below user.slice, and a scope unit below it for each concurrent session of a user. Also, a per-user service manager is started as system service instance of user@.service for each logged in user.
Generating and managing session IDs. If auditing is available and an audit session ID is already set for a session, then this ID is reused as the session ID. Otherwise, an independent session counter is used.
Providing polkit-based access for users for operations such as system shutdown or sleep
Implementing a shutdown/sleep inhibition logic for applications
Handling of power/sleep hardware keys
Session switch management
Device access management for users
Automatic spawning of text logins (gettys) on virtual console activation and user runtime directory management
User sessions are registered with logind via the pam_systemd(8) PAM module.
See logind.conf(5) for information about the configuration of this service.
See sd-login(3) for information about the basic concepts of logind such as users, sessions and seats.
See org.freedesktop.login1(5) and org.freedesktop.LogControl1(5) for information about the D-Bus APIs systemd-logind provides.
For more information on the inhibition logic see the Inhibitor Lock Developer Documentation.
If you are interested in writing a display manager that makes use of logind, please have look at Writing Display Managers. If you are interested in writing a desktop environment that makes use of logind, please have look at Writing Desktop Environments.
Inhibitor Lock Developer Documentation
Writing Display Managers
Writing Desktop Environments
systemd(1), systemd-user-sessions.service(8), loginctl(1), logind.conf(5), pam_systemd(8), sd-login(3)