Fast user switching

Fast user switching is a feature, i.e. a term for some rather loosely specified functionality, of a multi-user operating system. It allows users to switch between user accounts on a single computer without quitting applications and logging out.

The Linux kernel's VT subsystem (please consult the articles Pseudoterminal, Linux console and Virtual console) dates back to 1993 and does not understand the concept of multiple "seats", meaning that of the up to 63 VTs, only one VT can be active at any given time. Despite this kernel limitation, multiseat is supported on GNU/Linux. The feature of "fast user switching" has less severe necessities then multi-seat does because the multiple users don't work simultaneously, and therefore simply works.

The most straight forward solution to elegant multi-seat are kmscon/systemd-consoled in combination with systemd-logind. The available desktop environments such as GNOME or KDE Software Compilation adapt their graphical login and session manager (e.g. GDM, SDDM, LightDM, etc.) to the underneath solution and have to be configured to implement fast user switching that way.

For installations with older environments, the functionality must be enabled in the appropriate configuration files then a hot key sequence such as CTRL-ALT-F8 is pressed. A separate login window will now appear and the second user can log in (or even the first user again). Alternatively, in the default install, new X sessions can be started at will by using different display parameters to have them run in different virtual terminals (e.g. "startx -- :1" or "X :1 -query localhost"). Again, hot key sequences allow the user switching to take place.

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Wikipedia