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

In the Junos CLI, the keystroke used to auto-complete user-defined variables is the Tab key. This functionality allows users to quickly complete variable names, making command entry more efficient and reducing the chances of typing errors. Auto-completion can be particularly useful when working with long or complex variable names, as it provides suggestions and automatically fills in the rest of the name once the key is pressed.

The Tab key enhances user productivity by streamlining the process of entering commands, which is especially beneficial in a command-line interface where efficiency is paramount. This feature supports a more intuitive workflow, allowing operators to verify and complete their input without needing to remember every detail of variable names.

Contextually, while the other options do represent keystrokes with specific functions in the CLI, they do not pertain to the completion of user-defined variables. The Ctrl + a command generally moves the cursor to the beginning of the line, Ctrl + u deletes the line from the cursor position to the beginning, and Shift + Spacebar does not have a function related to auto-completion in the Junos environment. The primary purpose of these commands is different from the Tab key, which is specifically designed for completion tasks in the command line interface.