What key should you press for command completion within a Linux shell?

In a Linux shell, pressing the TAB key is the standard method for command completion. This feature allows users to start typing a command or a file name and then press TAB to automatically complete it if there is an unambiguous match. This functionality significantly speeds up the command entry process and helps reduce typing errors. For example, if a user types the beginning of a command, such as "ls" and presses TAB, the shell will either complete the command if unique or show possible completions if multiple matches exist.

Using the TAB key enhances productivity and efficiency, especially when working with long commands or file paths. Other keys, such as Enter, Space, and Ctrl, do not serve this specific purpose in the context of command completion in the shell. Therefore, the TAB key stands out as the correct answer in this scenario.