Terminal Concepts and Standards

Overview

This lecture explains the fundamental concepts of computer terminals, shells, terminal emulators, PTYs, and related standards, focusing on their evolution and how modern systems interact with them.

What is a Terminal?

• A computer terminal is a device for entering and displaying data to a computer system.
• Early terminals connected via serial lines and often output data to paper.
• Modern UNIX systems manage terminals using the TTY (Teletype) subsystem, which handles input/output streams.

Shells and Terminal Communication

• Terminals and TTYs only move bytes; they require a shell program (like bash or zsh) to interpret input and generate output.
• Shells interact with terminals indirectly through the kernel's TTY interface.

ANSI, ECMA-48, and Terminal Standards

• Early terminals used ASCII plus proprietary codes for features like bold text.
• ANSI created the X3.64 standard, later published freely as ECMA-48, to standardize terminal control codes (escape sequences).

Terminfo and Termcap Databases

• Termcap and terminfo databases map terminal functions (like bold or color) to their required byte sequences.
• Applications use these databases, referenced by the TERM environment variable, to produce correct output for different terminals.

Running Programs in a Terminal

• Shells start new programs with the TTY's input/output streams, letting those programs directly interact with the terminal.
• Programs must respect the TERM setting and use proper libraries for terminal output.

stdin, stdout, and stderr in Terminals

• UNIX maps stdin to TTY input, and both stdout and stderr to TTY output.
• From the terminal's perspective, there's only a single output stream, not separate stdout and stderr.

Foreground Processes and Job Control

• Only one process is usually "foreground" and interacts with the TTY, but careful coordination is required to avoid output conflicts.
• Shells can manage which process is foreground for signal delivery via job control.

Signals and Keyboard Shortcuts

• The TTY layer translates keystrokes like CTRL-C (0x03) into signals (SIGINT) sent to the foreground process.
• Shells handle these signals for interrupting or suspending processes (e.g., CTRL-Z for suspension).

Terminal Emulators and PTYs

• PTYs (Pseudo Terminal TeletYpes) allow software terminals not tied to physical devices.
• Terminal emulators create PTYs and spawn shell programs, interpreting data to display and encode user input.

Windows and ConPTY

• Traditional Windows terminals worked differently and lacked PTYs, limiting interoperability.
• Newer Windows versions include ConPTY, a pseudo-terminal interface with additional complexity to support legacy console APIs.
• Terminal emulators on Windows may use helpers like openconsole.exe or conhost.exe for PTY support.

Key Terms & Definitions

• Terminal — Hardware or software for input/output to a computer, historically via serial lines.
• TTY (Teletype) — Kernel subsystem managing terminal input/output streams in UNIX.
• Shell — A program like bash that interprets user commands and manages processes.
• ANSI Escape Sequences — Standardized byte codes for controlling terminal display features.
• Termcap/Terminfo — Databases mapping terminal features to their required codes.
• PTY (Pseudo Terminal) — Virtual device providing terminal-like interfaces to software.
• Terminal Emulator — Software that mimics the behavior of hardware terminals within graphical environments.
• ConPTY — The Windows implementation of pseudo-terminals for modern terminal emulation.

Action Items / Next Steps

• Review the terms and standards covered (TTY, PTY, ANSI, terminfo) and how they relate.
• Explore terminal emulators and practice using shell commands with respect to terminal capabilities.
• Optional: Read about ConPTY for deeper understanding of Windows terminal emulation.