XSessionID

XSessionID is a unique identifier generated by the X Window System to distinguish between different X sessions. An X session typically refers to a single graphical user interface instance running on a display. When a user logs into a graphical environment, or when a new X server starts, a new XSessionID is created. This ID is crucial for client applications to connect to the correct X server and establish a graphical session.

The XSessionID is usually a numerical value. X client applications, such as terminals, web browsers, or any graphical program, use this ID to locate and communicate with the X server. This communication happens through the X protocol. The X server manages the display, keyboard, and mouse input and output, and it uses the XSessionID to differentiate between multiple potential clients requesting access to its resources.

In practice, the XSessionID is often used in conjunction with other environment variables, most notably DISPLAY. The DISPLAY variable typically includes the hostname and display number, and the XSessionID can be considered an additional layer of identification for the specific session on that display. While not always directly exposed to the end-user, it plays a fundamental role in the underlying mechanics of how graphical applications interact with the display server in Unix-like operating systems. Its primary purpose is to ensure that the correct graphical environment is being targeted and managed.