Pseudoterminal

A pseudoterminal (PTY) is a pair of virtual devices that provides a terminal interface to programs, enabling software to interact with another process as if it were a physical terminal. The pair consists of a master and a slave; the master is controlled by a terminal-emulating process, while the slave behaves like a terminal device for the other process.

Data flows between the two ends: input written to the master is delivered to the slave as if typed by a user, and output produced by the slave appears on the master side. The kernel applies terminal settings (via termios) and simulates line discipline, echo, and other terminal features.

On Unix-like systems, the master is created with /dev/ptmx, and a corresponding slave device is exposed under /dev/pts/N. Utilities such as posix_openpt, grantpt, unlockpt, and ptsname are used to create and locate the slave. Terminal emulators connect to the master to present an interactive interface to users; the process running in the slave can use the terminal as its controlling terminal. In Windows, similar capabilities exist via ConPTY or pseudo consoles.

Common uses include terminal emulators, remote shells accessed via SSH, and terminal multiplexers like tmux or screen, which spawn shells inside PTYs for multiple synchronized sessions. PTYs also enable automated tools to interact with programs that expect a terminal.

Background: Pseudoterminals are part of the Unix TTY subsystem and differ from physical serial terminals by being software-emulated. They provide the mechanism by which programs can offer interactive text interfaces in a controlled, process-owned environment.