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Terminals, TTY, PTY, and ANSI Escape Codes

Chapters *0:00* - Introduction to Terminals, PTY, and TTY *0:08* - Running Commands and the Role of the Shell (e.g., Zsh, Bash) *1:50* - What is TTY (Teletypewriter)? *2:09* - Virtual Consoles and Terminal Emulators *2:41* - Pseudo-Terminals (PTY) and Terminal Simulation *2:50* - ANSI Escape Codes for Formatting (Color, Underline, Bold) *3:57* - Interpretation of ANSI Codes by Terminal Emulators *4:46* - Parsing ANSI Codes (Example with `pyte` and HTML) *6:29* - Processes, Controlling Terminals, and Signals (Ctrl+C, SIGHUP) *7:47* - How PTY Works and Why it's Needed *8:32* - Line Discipline: Cooked Mode (Canonical) vs. Raw Mode *9:40* - Line Discipline: Echoing *9:53* - Changing Terminal Options with `stty` (Disabling Canonical Mode and Echoing) *10:41* - Signal Management and Flow Control (Ctrl+S, Ctrl+Q) *11:49* - Window Size and Resizing Events (SIGWINCH) *13:47* - PTY and Remote Connections (SSH and PTY) *14:58* - Summary *** Blog post: https://cefboud.com/posts/terminals-p... Terminal Fundamentals: Shell, TTY, and PTY [cite_start]When you open a terminal window, you're running a *terminal emulator**[cite: 22]. [cite_start]The program that reads and executes your commands is the **shell* (e.g., Zsh, Bash, Fish)[cite: 9, 10, 11]. [cite_start]The shell parses the command, finds the executable file, and runs it within a child process[cite: 12, 13, 14]. [cite_start]The output is then displayed on the screen[cite: 15]. [cite_start]**TTY (Teletypewriter)**: This is the term used to refer to any terminal, whether physical or virtual[cite: 17]. [cite_start]**Virtual consoles** are TTYs built into the Linux kernel that operate without a graphical user interface (GUI)[cite: 19]. [cite_start]**PTY (Pseudo-Terminal)**: This is an OS functionality that simulates the behavior of a terminal[cite: 21]. [cite_start]Terminal emulators rely on PTYs[cite: 21]. [cite_start]PTYs act as a pipe with two ends, one for the terminal emulator and one for the command, making the command think it's connected to an actual terminal[cite: 57, 58]. *** ANSI Escape Codes [cite_start]**ANSI escape codes** are sequences that instruct the terminal how to format the words being displayed[cite: 68, 70, 71]. These codes allow for: [cite_start]Coloring text[cite: 72]. [cite_start]Underlining and bolding text[cite: 73, 74]. [cite_start]Moving the cursor and clearing the screen[cite: 77, 88]. [cite_start]Making sounds (e.g., the bell sound using $\text{NC code 7}$)[cite: 80, 81]. [cite_start]The terminal emulator itself is responsible for interpreting these codes[cite: 89, 90, 91]. *** Terminal Behavior and Line Discipline Commands run in a terminal are attached to a **controlling terminal**. The PTY/TTY functionality provides essential terminal behavior through a feature called **line discipline**. Key aspects of terminal behavior offered by TTY: *Buffering (Canonical/Cooked Mode)**: Input characters are buffered and only sent to the process when the user hits Enter. Programs like text editors can switch to **raw mode* to receive characters as they are typed. **Echoing**: When you type, the TTY module echoes the characters back to the screen so you can see them. **Signal Management**: The terminal handles signals like the interrupt signal ($\text{SIGINT}$) sent with $\text{Ctrl}+\text{C}$ to terminate a process. If a parent terminal closes, a hangup signal ($\text{SIGHUP}$) is sent to terminate all linked processes. **Flow Control**: The TTY subsystem manages flow control, such as blocking standard output with $\text{Ctrl}+\text{S}$ and resuming with $\text{Ctrl}+\text{Q}$. **Window Size/Resize**: The terminal has a size in rows and columns. When the window is resized, a $\text{SIGWINCH}$ signal is dispatched to programs so they can adjust. PTYs are crucial for modern applications like **SSH**, where a pseudo-terminal is started on the remote server to simulate a local terminal session and handle input/output and resize events over the network.