Terminal emulator in the context of "GNOME Terminal"

In computing, text-based user interfaces (TUI) (alternately terminal user interfaces, to reflect a dependence upon the properties of computer terminals and not just text), is a retronym describing a type of user interface (UI) common as an early form of human–computer interaction, before the advent of bitmapped displays and modern conventional graphical user interfaces (GUIs). Like modern GUIs, they can use the entire screen area and may accept mouse and other inputs. They may also use color and often structure the display using box-drawing characters such as ┌ and ╣. The modern context of use is usually a terminal emulator.

Box-drawing characters, also known as line-drawing characters, are a form of semigraphics widely used in text user interfaces to draw various geometric frames and boxes. These characters are characterized by being designed to be connected horizontally and/or vertically with adjacent characters, which requires proper alignment. Box-drawing characters therefore typically only work well with monospaced fonts.

In graphical user interfaces, these characters are much less useful as it is simpler to draw lines and rectangles directly with graphical APIs. However, they are still useful for command-line interfaces and plaintext comments within source code.

A "Unix shell" is a shell that provides a command-line user interface for a Unix-like operating system. A Unix shell provides a command language that can be used either interactively or for writing a shell script. A user typically works within a Unix shell via a terminal emulator; however, direct access via serial hardware connections or a Secure Shell are common for server systems. Although use of a Unix shell is popular with some users, others prefer to use a graphical shell in a windowing system, such as those provided in desktop Linux distributions or macOS, instead of a command-line interface (CLI).

A user may have access to multiple Unix shells with one configured to run by default when the user logs in interactively. The default selection is typically stored in a user's profile (for example, in the local passwd file or in a distributed configuration system such as NIS or LDAP). A user may use other shells nested inside the default shell.

An operating system shell is a computer program that provides relatively broad and direct access to the system on which it runs. The term shell refers to how it is a relatively thin layer around an operating system.

Most shells are command-line interface (CLI) programs. Some graphical user interfaces (GUI) also include shells.

A bell character (sometimes bell code) is a device control code originally sent to ring a small electromechanical bell on tickers and other teleprinters and teletypewriters to alert operators at the other end of the line, often of an incoming message. Though tickers punched the bell codes into their tapes, printers generally do not print a character when the bell code is received. Bell codes are usually represented by the label BEL. They have been used since 1870 (initially in the Baudot code).

To maintain backward compatibility, video display terminals (VDTs) that replaced teletypewriters included speakers or buzzers to perform the same function, as did the personal computers that followed. Modern terminal emulators often integrate the warnings to the desktop environment (e.g., the macOS Terminal will play the system warning sound) and also often offer a silent visual bell feature that flashes the terminal window briefly.