Introduction

A student new to Systems Programming will eventually stumble upon a strange method for printing color in a terminal. They might see something like the Arch Linux Color Bash Prompt guide, introducing a table of shell variables that appear something like a markup language for:

magenta = '\x1b[0;35m'

print(magenta + 'magenta is an original primary CGA color.')

In this article we will examine how the magic string \x1b[0;35m may be constructed with some determinism. Then, view the source code of xterm(1) to learn how they are interpreted. Finally, we will look at a class of sequences parsed by xterm(1) that elicit a response from your terminal emulator.

Capabilities

If we review the manual for terminfo(5):

$ man 5 terminfo

We discover a database of terminal capabilities that allows us to construct these special sequence strings. If we dig deeper, we will find a parameterized language for describing terminal capabilities.

The termcap.src file is authored in a special language that helps associate terminals defined by TERM environment value to their terminal capability strings.

With the curses module of python, we can access the C library routines that for the capabilities database defined by terminfo(5):

import curses
curses.setupterm()

cyan = curses.tparm(curses.tigetstr('setaf'),
                    curses.COLOR_CYAN).decode()

print(cyan + 'cyan is a primary CGA color.')

The blessed library provides a much simpler interface:

import blessed
term = blessed.Terminal()

print(term.red + 'red was not introduced until EGA.')

Although you are welcome to print raw strings directly to the user's terminal, as often recommended by introductory guides, using the terminfo(5) database ensures the correct sequences for the given user's TERM environment value are used. It also allows the Operating System to maintain terminal support independently of your software.

Rendering

The ASCII control character ESCAPE (\x1b) begins a detour to a special processing routine when received by a terminal emulator. The string phrase '\x1b[1;35m' is meaningful as a kind of markup language.

We can read this as a standard polish notation parser: placing arguments onto the stack, then calling the defining function:

• \x1b[ is the Control Sequence Inducer (CSI) sequence,
• followed by parameters 1;31 (Bold, Magenta),
• with final function m for Select Graphics Rendition (SGR).

2673                 case 31:
(...)
2679                 case 37:
2680                     if_OPT_ISO_COLORS(screen, {
2681                         xw->sgr_foreground = (op - 30);
2682                         xw->sgr_extended = False;
2683                         setExtendedFG(xw);
2684                     });
3685                     break;

print('\x1b#8')

printf "\x1b(0\x1b)B"

printf "\x1bc"

Reactor

Applications may write hidden messages that change the state of your terminal, but they may also request your terminal emulator to write hidden messages in return!

Let's try one, Report Cursor Position:

$ printf "\x1b[6n"; read input
$ set | grep ^input
input=$'\E[38;1R'

This is a feature of the blessed library:

import blessed
term = blessed.Terminal()

print(term.get_location())

There are other sequences that cause a terminal emulator to write a response, some terminals respond to the raw control character, ^E (\x05) with a terminal identifier, such as PuTTY.

whatis.telnet.org

An upcoming project will be an interactive, fingerprinting telnet server. It will produce a private report of all of the details it was able to retrieve, and hosted at telnet address whatis.telnet.org.