Linux TTY
External
Internal
Overview
"TTY" is a Linux subsystem that provides the conduit through which keyboard strokes captured while the focus is in a terminal such as iTerm2 or xterm are turned into the right characters and delivered to UNIX processes. At the same time, the TTY subsystem funnels the process' generated output back to the terminal. The name comes from "teletype", which used to be electro-mechanical machines that read keyboard entires, turned them into electrical signals and sent them across the wire to to other teletypes.
TTY Functions
Line Editing
TTY subsystem provide line editing services, where a backspace key actually removes the last typed character from an internal editing buffer, instead of being sent to the user process. This functionality is implemented by a component known as line discipline. Line discipline also contain options for character echoing, automatic conversion between CR and LF. User processes have the choices of disabling this functionality, by putting the line discipline in "raw" mode, instead of the default "cooked" (or "canonical") mode. The kernel provides several line disciplines, but only one is attached to a given serial device at a time.
Session Management
TTY session management allows one user to interact with multiple processes running simultaneously in user space, one at a time. This includes the capability to suspend, kill or send programs to background, bring programs into foreground, and direct the user input to the foreground process only. The TTY subsystem implements these features in the TTY driver.
Components
The TTY subsystem has several kernel components:
Line Discipline
A kernel-level component providing line editing capabilities.
TTY Driver
The TTY Driver is a kernel component that implements session management. The TTY driver does not have an execution context - it has data fields and methods, but the data gets updated only if the methods get called by a process or a kernel interrupt handler.
Pseudo tty
With the first terminals, there was always a piece of hardware (display or serial port) attached to the device. With XWindows, telnet and ssh, there came a need for software pseudo devices to do the job of standing in for display hardware. These are the pseudo terminals, software that emulates terminal hardware, handling input and output in the same way a physical device would do.