Hoofdstuk 8. Basis Configuratie: Netwerk, Accounts, Printen...

Historically, each locale has an associated “character set” (group of known characters) and a preferred “encoding” (internal representation for characters within the computer).

The most popular encodings for latin-based languages were limited to 256 characters because they opted to use a single byte for each character. Since 256 characters was not enough to cover all European languages, multiple encodings were needed, and that is how we ended up with ISO-8859-1 (also known as “Latin 1”) up to ISO-8859-15 (also known as “Latin 9”), among others.

Working with foreign languages often implied regular switches between various encodings and character sets. Furthermore, writing multilingual documents led to further, almost intractable problems. Unicode (a super-catalog of nearly all writing systems from all of the world's languages) was created to work around this problem. One of Unicode's encodings, UTF-8, retains all 128 ASCII symbols (7-bit codes), but handles other characters differently. Those are preceded by a specific escape sequence of a few bits, which implicitly defines the length of the character. This allows encoding all Unicode characters on a sequence of one or more bytes. Its use has been popularized by the fact that it is the default encoding in XML documents.

This is the encoding that should generally be used, and is thus the default on Debian systems.

The /etc/environment file provides the login, gdm, or even ssh programs with the correct environment variables to be created.

These applications do not create these variables directly, but rather via a PAM (pam_env.so) module. PAM (Pluggable Authentication Module) is a modular library centralizing the mechanisms for authentication, session initialization, and password management. See Paragraaf 11.7.3.2, “Configuring PAM” for an example of PAM configuration.

The /etc/default/locale file works in a similar manner, but contains only the LANG environment variable. Thanks to this split, some PAM users can inherit a complete environment without localization. Indeed, it is generally discouraged to run server programs with localization enabled; on the other hand, localization and regional settings are recommended for programs that open user sessions.

When a text is sent (or stored) without encoding information, it is not always possible for the recipient to know with certainty what convention to use for determining the meaning of a set of bytes. You can usually get an idea by getting statistics on the distribution of values present in the text, but that doesn't always give a definite answer. When the encoding system chosen for reading differs from that used in writing the file, the bytes are misinterpreted, and you get, at best, errors on some characters, or, at worst, something completely illegible.

Thus, if a French text appears normal with the exception of accented letters and certain symbols which appear to be replaced with sequences of characters like “é” or è” or “ç”, it is probably a file encoded as UTF-8 but interpreted as ISO-8859-1 or ISO-8859-15. This is a sign of a local installation that has not yet been migrated to UTF-8. If, instead, you see question marks instead of accented letters — even if these question marks seem to also replace a character that should have followed the accented letter — it is likely that your installation is already configured for UTF-8 and that you have been sent a document encoded in Western ISO.

So much for “simple” cases. These cases only appear in Western culture, since Unicode (and UTF-8) was designed to maximize the common points with historical encodings for Western languages based on the Latin alphabet, which allows recognition of parts of the text even when some characters are missing.

In more complex configurations, which, for example, involve two environments corresponding to two different languages that do not use the same alphabet, you often get completely illegible results — a series of abstract symbols that have nothing to do with each other. This is especially common with Asian languages due to their numerous languages and writing systems. The Japanese word mojibake has been adopted to describe this phenomenon. When it appears, diagnosis is more complex and the simplest solution is often to simply migrate to UTF-8 on both sides.