recurse-talks/terminal-escapes.md

#!/bin/env slides

Demystifying Terminal Escapes

by Bruce Hill

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Have you ever wondered how Vim and other command-line programs manage to do cool stuff with your terminal?

Like drawing all over the place without cluttering your command line history?

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Well, it's not magic, and you can do it too!

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There are two main topics I'll cover:

1. Printing terminal escape sequences
2. Getting and setting terminal information

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Most terminal emulators that you use (iTerm, alacritty, GNOME terminal, etc) are emulating old-school physical terminals like the VT100:

firefox --new-window https://en.wikipedia.org/wiki/VT100

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The way that terminals used to work is that you would send bytes from a computer over a wire to a terminal (the termination point of the signal).

Commonly Sent

• Text to print
• Escape sequences

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Escape Sequences

In order to do stuff like move the cursor around, clear the screen, etc. you could send an escape sequence to the terminal.

One common escape sequence you might know is the newline \n and carriage return \r escapes. These moved the carriage on a teletype to the next line or back to the start of a line.

term

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Operating System Commands

More advanced escape sequences like Operating System Commands let you do more fancy stuff like moving the cursor position to an arbitrary X/Y position or setting the text color.

The magic Operating System Command is two bytes:

\033[

Also known as \x1B[ or \e[.

It tells the terminal that the next thing that follows isn't text to print but is a command that needs to run.

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Here are some fun operating system commands:

ansi | less -R

You can run these yourself with just a print statement!

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Here's what's actually getting sent to the terminal:

ansi | less

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Alternate Screens

One of the most important operating system commands is toggling the alternate screen on and off:

Alternate screen on:
\033[?1049h

Alternate screen off:
\033[?1049l

This is what lets you run a terminal program that doesn't spam the scrollback log!

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Getting and Setting Info

In C, there are some functions that let you get information about the terminal

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tcgetattr/tcsetattr

Useful for reading one keypress at a time!

int tcgetattr(int fd, struct termios *termios_p);
int tcsetattr(int fd, int optional_actions,
              const struct termios *termios_p);
void cfmakeraw(struct termios *termios_p);

man tcsetattr

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ioctl

Useful for getting terminal width/height!

int ioctl(int fd, int op, ...);

man ioctl

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BTUI

I wrote a library called BTUI to do the bare minimum of this stuff:

firefox --new-window https://github.com/bruce-hill/btui

(I really don't like ncurses, it's very bloated and unwieldy)

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slides

This presentation is made using the BTUI library and its Python bindings.

Thanks to Michael Rees for the idea to make a terminal slide presenter!

firefox --new-window https://github.com/bruce-hill/slides

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Goodbye!

Thanks for your time! I'm Bruce Hill if you want to find me on zulip.

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Bonus Demo 1: Conway's Game of Life

conway

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Bonus Demo 2: Double Pendulum

2pend

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Bonus Demo 3: Space Invaders

cd ~/software/btui/Python/invaders
./invaders.py

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Bonus Demo 4: Alternate Character Set

alternate-character-set | less -r