3.5 What are pttys?

Pseudo-teletypes (pttys, ptys, other variant abbreviations) are pseudo-devices that have two parts: the master side, which can be thought of as the `user', and the slave side, which behaves like a standard tty device.

They exist in order to provide a means to emulate the behaviour of a serial terminal under the control of a program. For example, telnet uses a pseudo-terminal on the remote system; the remote login shell sees the behaviour it expects from a tty device, but the master side of the pseudo-terminal is being controlled by a daemon that forwards all data over the network. They are also used by programs such as xterm, expect, script, screen, emacs, and many others.

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