POSIX terminal interface

The POSIX terminal interface is the generalized abstraction, comprising both an Application Programming Interface for programs, and a set of behavioural expectations for users of a terminal, as defined by the POSIX standard and the Single Unix Specification. It is a historical development from the terminal interfaces of BSD version 4 and Seventh Edition Unix.

A multiplicity of I/O devices are regarded as "terminals" in Unix systems. These include:

Unlike its mainframe and minicomputer contemporaries, the original Unix system was developed solely for dumb terminals, and that remains the case today. A terminal is a character-oriented device, comprising streams of characters received from and sent to the device. Although the streams of characters are structured, incorporating control characters, escape codes, and special characters, the I/O protocol is not structured as would be the I/O protocol of smart, or intelligent, terminals. There are no field format specifications. There's no block transmission of entire screens (input forms) of input data.

By contrast mainframes and minicomputers in closed architectures commonly use Block-oriented terminals.

The "capabilities" of a terminal comprise various dumb terminal features that are above and beyond what is available from a pure teletypewriter, which programs can make use of. They (mainly) comprise escape codes that can be sent to or received from the terminal. The escape codes sent to the terminal perform various functions that a CRT terminal (or software terminal emulator) is capable of that a teletypewriter is not, such as moving the terminal's cursor to positions on the screen, clearing and scrolling all or parts of the screen, turning on and off attached printer devices, programmable function keys, changing display colours and attributes (such as reverse video), and setting display title strings. The escape codes received from the terminal signify things such as function key, arrow key, and other special keystrokes (home key, end key, help key, PgUp key, PgDn key, insert key, delete key, and so forth).

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