Enter the void *

Unicode : Math, greek, symbols - you name it !

by Etienne Millon on November 28, 2011

Tagged as: linux.

EBCDIC, ASCII & the power of legacy

… and no, that’s not a movie title.

As you know, all your computer knows about is numbers, yet when you type on a keyboard, a character appears on your screen. This is thanks to character encodings.

There are several norms that defines how characters (ie, glyphs) are encoded into numbers. Besides dinosaurs such as EBCDIC, the “classic” way of encoding is ASCII – that is what most modern1 operating systems use internally.

The problem with ASCII is that it maps every character to a single byte with MSB reset, meaning that you can have a maximum of 128 glyphs. It’s “good enough” for English (hey, the A stands for American) but terrible for international characters. This is even worse considering that 32 of them are control characters, ie mostly legacy. Did you ever need to interpret DC2, SI or GS in a program ?

The eighth bit being “reserved” can be used to support “extended characters”. Several vendors (including Microsoft) used the concept of “code pages” to use extra glyphs in the 128-255 range. For example, Latin-1 was used in western europe to display accentuated characters.

If all your data comes from one part of the world, it works fine, but with the following limitations if you need to handle international data :

In other words, a more extensible system is needed. Hopefully, this system exists and is called…


Unicode separates two notions :

What’s nice is that it’s easy to enter Unicode under X11. The last two sections explain how you can configure your system to type (for example) √, β and ✈ !

Configure a compose key

A “compose” key, or Multi_key under X11, will begin a character compose sequence. For example, when I type <Multi_key> <s> <q>, a square root (U+221A √) is entered.

To configure a compose key, you can use xmodmap(1). Put the following into ~/.Xmodmap to make your right control key act as a Multi_key :

keysym Control_R = Multi_key

Unfortunately, this file is not loaded automatically, so you have to run xmodmap ~/.Xmodmap when opening a X session (this can be done automatically if you put in in your ~/.xsession, for example).

Define a .XCompose mapping

The second part is to define mappings between key sequences and unicode codepoints. This is the role of the ~/.XCompose file.

As described in xcompose(5), a line looks like :

<Multi_key> <ampersand> <p> <l> <a> <n> <e>     : "✈"   U2708     # AIRPLANE

ie, a key sequence, a colon, a string and a character name. The comment does not hurt, as usual.

To start your own list of bindings, I suggest kragen’s repository, which includes an excellent set. And if you need to find a specific unicode character, the unicode script is very useful !

TL;DR: Spread the word, ♥ Unicode ☺

  1. Yes, that excludes AS/400.