charsets(7) Miscellaneous Information Manual charsets(7)
NAME
charsets - character set standards and internationalization
DESCRIPTION
This manual page gives an overview on different character set standards
and how they were used on Linux before Unicode became ubiquitous. Some
of this information is still helpful for people working with legacy sys-
tems and documents.
Standards discussed include such as ASCII, GB 2312, ISO/IEC 8859, JIS,
KOI8-R, KS, and Unicode.
The primary emphasis is on character sets that were actually used by lo-
cale character sets, not the myriad others that could be found in data
from other systems.
ASCII
ASCII (American Standard Code For Information Interchange) is the origi-
nal 7-bit character set, originally designed for American English. Also
known as US-ASCII. It is currently described by the ISO/IEC 646:1991
IRV (International Reference Version) standard.
Various ASCII variants replacing the dollar sign with other currency
symbols and replacing punctuation with non-English alphabetic characters
to cover German, French, Spanish, and others in 7 bits emerged. All are
deprecated; glibc does not support locales whose character sets are not
true supersets of ASCII.
As Unicode, when using UTF-8, is ASCII-compatible, plain ASCII text
still renders properly on modern UTF-8 using systems.
ISO/IEC 8859
ISO/IEC 8859 is a series of 15 8-bit character sets, all of which have
ASCII in their low (7-bit) half, invisible control characters in posi-
tions 128 to 159, and 96 fixed-width graphics in positions 160–255.
Of these, the most important is ISO/IEC 8859-1 ("Latin Alphabet No. 1" /
Latin-1). It was widely adopted and supported by different systems, and
is gradually being replaced with Unicode. The ISO/IEC 8859-1 characters
are also the first 256 characters of Unicode.
Console support for the other ISO/IEC 8859 character sets is available
under Linux through user-mode utilities (such as setfont(8)) that modify
keyboard bindings and the EGA graphics table and employ the "user map-
ping" font table in the console driver.
Here are brief descriptions of each character set:
ISO/IEC 8859-1 (Latin-1)
Latin-1 covers many European languages such as Albanian, Basque,
Danish, English, Faroese, Galician, Icelandic, Irish, Italian,
Norwegian, Portuguese, Spanish, and Swedish. The lack of the
ligatures Dutch IJ/ij, French œ, and „German“ quotation marks was
considered tolerable.
ISO/IEC 8859-2 (Latin-2)
Latin-2 supports many Latin-written Central and East European
languages such as Bosnian, Croatian, Czech, German, Hungarian,
Polish, Slovak, and Slovene. Replacing Romanian ș/ț with ş/ţ was
considered tolerable.
ISO/IEC 8859-3 (Latin-3)
Latin-3 was designed to cover of Esperanto, Maltese, and Turkish,
but ISO/IEC 8859-9 later superseded it for Turkish.
ISO/IEC 8859-4 (Latin-4)
Latin-4 introduced letters for North European languages such as
Estonian, Latvian, and Lithuanian, but was superseded by
ISO/IEC 8859-10 and ISO/IEC 8859-13.
ISO/IEC 8859-5
Cyrillic letters supporting Bulgarian, Byelorussian, Macedonian,
Russian, Serbian, and (almost completely) Ukrainian. It was
never widely used, see the discussion of KOI8-R/KOI8-U below.
ISO/IEC 8859-6
Was created for Arabic. The ISO/IEC 8859-6 glyph table is a
fixed font of separate letter forms, but a proper display engine
should combine these using the proper initial, medial, and final
forms.
ISO/IEC 8859-7
Was created for Modern Greek in 1987, updated in 2003.
ISO/IEC 8859-8
Supports Modern Hebrew without niqud (punctuation signs). Niqud
and full-fledged Biblical Hebrew were outside the scope of this
character set.
ISO/IEC 8859-9 (Latin-5)
This is a variant of Latin-1 that replaces Icelandic letters with
Turkish ones.
ISO/IEC 8859-10 (Latin-6)
Latin-6 added the Inuit (Greenlandic) and Sami (Lappish) letters
that were missing in Latin-4 to cover the entire Nordic area.
ISO/IEC 8859-11
Supports the Thai alphabet and is nearly identical to the TIS-620
standard.
ISO/IEC 8859-12
This character set does not exist.
ISO/IEC 8859-13 (Latin-7)
Supports the Baltic Rim languages; in particular, it includes
Latvian characters not found in Latin-4.
ISO/IEC 8859-14 (Latin-8)
This is the Celtic character set, covering Old Irish, Manx,
Gaelic, Welsh, Cornish, and Breton.
ISO/IEC 8859-15 (Latin-9)
Latin-9 is similar to the widely used Latin-1 but replaces some
less common symbols with the Euro sign and French and Finnish
letters that were missing in Latin-1.
ISO/IEC 8859-16 (Latin-10)
This character set covers many Southeast European languages, and
most importantly supports Romanian more completely than Latin-2.
KOI8-R / KOI8-U
KOI8-R is a non-ISO character set popular in Russia before Unicode. The
lower half is ASCII; the upper is a Cyrillic character set somewhat bet-
ter designed than ISO/IEC 8859-5. KOI8-U, based on KOI8-R, has better
support for Ukrainian. Neither of these sets are ISO/IEC 2022 compati-
ble, unlike the ISO/IEC 8859 series.
Console support for KOI8-R is available under Linux through user-mode
utilities that modify keyboard bindings and the EGA graphics table, and
employ the "user mapping" font table in the console driver.
GB 2312
GB 2312 is a mainland Chinese national standard character set used to
express simplified Chinese. Just like JIS X 0208, characters are mapped
into a 94x94 two-byte matrix used to construct EUC-CN. EUC-CN is the
most important encoding for Linux and includes ASCII and GB 2312. Note
that EUC-CN is often called as GB, GB 2312, or CN-GB.
Big5
Big5 was a popular character set in Taiwan to express traditional Chi-
nese. (Big5 is both a character set and an encoding.) It is a superset
of ASCII. Non-ASCII characters are expressed in two bytes. Bytes
0xa1–0xfe are used as leading bytes for two-byte characters. Big5 and
its extension were widely used in Taiwan and Hong Kong. It is not
ISO/IEC 2022 compliant.
JIS X 0208
JIS X 0208 is a Japanese national standard character set. Though there
are some more Japanese national standard character sets (like JIS X
0201, JIS X 0212, and JIS X 0213), this is the most important one.
Characters are mapped into a 94x94 two-byte matrix, whose each byte is
in the range 0x21–0x7e. Note that JIS X 0208 is a character set, not an
encoding. This means that JIS X 0208 itself is not used for expressing
text data. JIS X 0208 is used as a component to construct encodings
such as EUC-JP, Shift_JIS, and ISO/IEC 2022-JP. EUC-JP is the most im-
portant encoding for Linux and includes ASCII and JIS X 0208. In EUC-
JP, JIS X 0208 characters are expressed in two bytes, each of which is
the JIS X 0208 code plus 0x80.
KS X 1001
KS X 1001 is a Korean national standard character set. Just as JIS X
0208, characters are mapped into a 94x94 two-byte matrix. KS X 1001 is
used like JIS X 0208, as a component to construct encodings such as EUC-
KR, Johab, and ISO/IEC 2022-KR. EUC-KR is the most important encoding
for Linux and includes ASCII and KS X 1001. KS C 5601 is an older name
for KS X 1001.
ISO/IEC 2022 and ISO/IEC 4873
The ISO/IEC 2022 and ISO/IEC 4873 standards describe a font-control
model based on VT100 practice. This model is (partially) supported by
the Linux kernel and by xterm(1). Several ISO/IEC 2022-based character
encodings have been defined, especially for Japanese.
There are 4 graphic character sets, called G0, G1, G2, and G3, and one
of them is the current character set for codes with high bit zero (ini-
tially G0), and one of them is the current character set for codes with
high bit one (initially G1). Each graphic character set has 94 or 96
characters, and is essentially a 7-bit character set. It uses codes ei-
ther 040–0177 (041–0176) or 0240–0377 (0241–0376). G0 always has size
94 and uses codes 041–0176.
Switching between character sets is done using the shift functions ^N
(SO or LS1), ^O (SI or LS0), ESC n (LS2), ESC o (LS3), ESC N (SS2), ESC
O (SS3), ESC ~ (LS1R), ESC } (LS2R), ESC | (LS3R). The function LSn
makes character set Gn the current one for codes with high bit zero.
The function LSnR makes character set Gn the current one for codes with
high bit one. The function SSn makes character set Gn (n=2 or 3) the
current one for the next character only (regardless of the value of its
high order bit).
A 94-character set is designated as Gn character set by an escape se-
quence ESC ( xx (for G0), ESC ) xx (for G1), ESC * xx (for G2), ESC + xx
(for G3), where xx is a symbol or a pair of symbols found in the
ISO/IEC 2375 International Register of Coded Character Sets. For exam-
ple, ESC ( @ selects the ISO/IEC 646 character set as G0, ESC ( A se-
lects the UK standard character set (with pound instead of number sign),
ESC ( B selects ASCII (with dollar instead of currency sign), ESC ( M
selects a character set for African languages, ESC ( ! A selects the
Cuban character set, and so on.
A 96-character set is designated as Gn character set by an escape se-
quence ESC - xx (for G1), ESC . xx (for G2) or ESC / xx (for G3). For
example, ESC - G selects the Hebrew alphabet as G1.
A multibyte character set is designated as Gn character set by an escape
sequence ESC $ xx or ESC $ ( xx (for G0), ESC $ ) xx (for G1), ESC $ *
xx (for G2), ESC $ + xx (for G3). For example, ESC $ ( C selects the
Korean character set for G0. The Japanese character set selected by ESC
$ B has a more recent version selected by ESC & @ ESC $ B.
ISO/IEC 4873 stipulates a narrower use of character sets, where G0 is
fixed (always ASCII), so that G1, G2, and G3 can be invoked only for
codes with the high order bit set. In particular, ^N and ^O are not
used anymore, ESC ( xx can be used only with xx=B, and ESC ) xx, ESC *
xx, ESC + xx are equivalent to ESC - xx, ESC . xx, ESC / xx, respec-
tively.
TIS-620
TIS-620 is a Thai national standard character set and a superset of
ASCII. In the same fashion as the ISO/IEC 8859 series, Thai characters
are mapped into 0xa1–0xfe.
Unicode
Unicode (ISO/IEC 10646) is a standard which aims to unambiguously repre-
sent every character in every human language. Unicode's structure per-
mits 20.1 bits to encode every character. Since most computers don't
include 20.1-bit integers, Unicode is usually encoded as 32-bit integers
internally and either a series of 16-bit integers (UTF-16) (needing two
16-bit integers only when encoding certain rare characters) or a series
of 8-bit bytes (UTF-8).
Linux represents Unicode using the 8-bit Unicode Transformation Format
(UTF-8). UTF-8 is a variable length encoding of Unicode. It uses 1
byte to code 7 bits, 2 bytes for 11 bits, 3 bytes for 16 bits, 4 bytes
for 21 bits, 5 bytes for 26 bits, 6 bytes for 31 bits.
Let 0,1,x stand for a zero, one, or arbitrary bit. A byte 0xxxxxxx
stands for the Unicode 00000000 0xxxxxxx which codes the same symbol as
the ASCII 0xxxxxxx. Thus, ASCII goes unchanged into UTF-8, and people
using only ASCII do not notice any change: not in code, and not in file
size.
A byte 110xxxxx is the start of a 2-byte code, and 110xxxxx 10yyyyyy is
assembled into 00000xxx xxyyyyyy. A byte 1110xxxx is the start of a
3-byte code, and 1110xxxx 10yyyyyy 10zzzzzz is assembled into xxxxyyyy
yyzzzzzz. (When UTF-8 is used to code the 31-bit ISO/IEC 10646 then
this progression continues up to 6-byte codes.)
For most texts in ISO/IEC 8859 character sets, this means that the char-
acters outside of ASCII are now coded with two bytes. This tends to ex-
pand ordinary text files by only one or two percent. For Russian or
Greek texts, this expands ordinary text files by 100%, since text in
those languages is mostly outside of ASCII. For Japanese users this
means that the 16-bit codes now in common use will take three bytes.
While there are algorithmic conversions from some character sets (espe-
cially ISO/IEC 8859-1) to Unicode, general conversion requires carrying
around conversion tables, which can be quite large for 16-bit codes.
Note that UTF-8 is self-synchronizing: 10xxxxxx is a tail, any other
byte is the head of a code. Note that the only way ASCII bytes occur in
a UTF-8 stream, is as themselves. In particular, there are no embedded
NULs ('\0') or '/'s that form part of some larger code.
Since ASCII, and, in particular, NUL and '/', are unchanged, the kernel
does not notice that UTF-8 is being used. It does not care at all what
the bytes it is handling stand for.
Rendering of Unicode data streams is typically handled through "subfont"
tables which map a subset of Unicode to glyphs. Internally the kernel
uses Unicode to describe the subfont loaded in video RAM. This means
that in the Linux console in UTF-8 mode, one can use a character set
with 512 different symbols. This is not enough for Japanese, Chinese,
and Korean, but it is enough for most other purposes.
SEE ALSO
iconv(1), ascii(7), iso_8859-1(7), unicode(7), utf-8(7)
Linux man-pages 6.9.1 2024-06-15 charsets(7)
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