screen
A terminal multiplexer that allows multiple shell sessions to run simultaneously within a single terminal window, with the ability to detach and reattach sessions.
screen is a utility that creates a virtual terminal environment, letting you run multiple independent shell sessions inside one physical terminal. You can switch between sessions, detach and leave processes running in the background, then reattach later without losing your work.
Common use cases include running long-running tasks (like downloads or compilations) that survive SSH disconnections, or organizing multiple related work sessions. For example, you might run screen -S backup to start a named session, detach with Ctrl+A then D, disconnect from your SSH session, and later reconnect and type screen -r backup to resume exactly where you left off.
Within a screen session, Ctrl+A is the escape key for commands: Ctrl+A C creates a new window, Ctrl+A N switches to the next window, and Ctrl+A ? shows help. Screen has largely been superseded by tmux, which offers similar functionality with more modern features and configuration options.