Which keystroke is used to auto-complete user-defined variables in the Junos CLI?

In the Junos CLI, the Tab key is used to auto-complete user-defined variables. This function is particularly useful for simplifying command entry and reducing the chances of typographical errors. When you start typing a variable name and press the Tab key, the CLI attempts to complete the command based on the context of what you have entered, allowing for a quicker and more efficient command-line experience. It is a common feature found in many command-line interfaces to assist users in navigating commands and variables without having to type out the entire name.

The other options provided do not serve the same purpose within the Junos CLI environment. For instance, while Ctrl + a and Ctrl + u have specific functionalities related to cursor movement and line editing, they do not contribute to auto-completion. Shift + Spacebar is also not associated with any commonly recognized function for auto-completing variables in the context of the Junos operating system. Thus, using the Tab key is essential for enhancing productivity and efficiency in the command-line interface.