1charsets(7)            Miscellaneous Information Manual            charsets(7)
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NAME

6       charsets - character set standards and internationalization
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DESCRIPTION

9       This manual page gives an overview on different character set standards
10       and how they were used on Linux before Unicode became ubiquitous.  Some
11       of  this  information  is  still helpful for people working with legacy
12       systems and documents.
13
14       Standards discussed include such as ASCII,  GB  2312,  ISO  8859,  JIS,
15       KOI8-R, KS, and Unicode.
16
17       The  primary  emphasis  is on character sets that were actually used by
18       locale character sets, not the myriad others that  could  be  found  in
19       data from other systems.
20
21   ASCII
22       ASCII (American Standard Code For Information Interchange) is the orig‐
23       inal 7-bit character set, originally  designed  for  American  English.
24       Also  known as US-ASCII.  It is currently described by the ISO 646:1991
25       IRV (International Reference Version) standard.
26
27       Various ASCII variants replacing the dollar sign  with  other  currency
28       symbols  and  replacing punctuation with non-English alphabetic charac‐
29       ters to cover German, French, Spanish, and others in  7  bits  emerged.
30       All are deprecated; glibc does not support locales whose character sets
31       are not true supersets of ASCII.
32
33       As Unicode, when using UTF-8, is  ASCII-compatible,  plain  ASCII  text
34       still renders properly on modern UTF-8 using systems.
35
36   ISO 8859
37       ISO  8859  is  a  series  of 15 8-bit character sets, all of which have
38       ASCII in their low (7-bit) half, invisible control characters in  posi‐
39       tions 128 to 159, and 96 fixed-width graphics in positions 160–255.
40
41       Of  these,  the  most important is ISO 8859-1 ("Latin Alphabet No. 1" /
42       Latin-1).  It was widely adopted and supported  by  different  systems,
43       and  is  gradually being replaced with Unicode.  The ISO 8859-1 charac‐
44       ters are also the first 256 characters of Unicode.
45
46       Console support for the other 8859 character sets  is  available  under
47       Linux through user-mode utilities (such as setfont(8)) that modify key‐
48       board bindings and the EGA graphics table and employ the "user mapping"
49       font table in the console driver.
50
51       Here are brief descriptions of each character set:
52
53       8859-1 (Latin-1)
54              Latin-1 covers many European languages such as Albanian, Basque,
55              Danish, English, Faroese, Galician, Icelandic,  Irish,  Italian,
56              Norwegian,  Portuguese,  Spanish,  and Swedish.  The lack of the
57              ligatures Dutch IJ/ij, French œ, and „German“ quotation marks  was
58              considered tolerable.
59
60       8859-2 (Latin-2)
61              Latin-2  supports  many  Latin-written Central and East European
62              languages such as Bosnian, Croatian, Czech,  German,  Hungarian,
63              Polish,  Slovak,  and  Slovene.  Replacing Romanian ș/ț with ş/ţ
64              was considered tolerable.
65
66       8859-3 (Latin-3)
67              Latin-3 was designed to cover of Esperanto, Maltese,  and  Turk‐
68              ish, but 8859-9 later superseded it for Turkish.
69
70       8859-4 (Latin-4)
71              Latin-4  introduced letters for North European languages such as
72              Estonian, Latvian, and Lithuanian, but was superseded by 8859-10
73              and 8859-13.
74
75       8859-5 Cyrillic letters supporting Bulgarian, Byelorussian, Macedonian,
76              Russian, Serbian, and (almost  completely)  Ukrainian.   It  was
77              never widely used, see the discussion of KOI8-R/KOI8-U below.
78
79       8859-6 Was  created for Arabic.  The 8859-6 glyph table is a fixed font
80              of separate letter forms, but a  proper  display  engine  should
81              combine these using the proper initial, medial, and final forms.
82
83       8859-7 Was created for Modern Greek in 1987, updated in 2003.
84
85       8859-8 Supports Modern Hebrew without niqud (punctuation signs).  Niqud
86              and full-fledged Biblical Hebrew were outside the scope of  this
87              character set.
88
89       8859-9 (Latin-5)
90              This  is  a  variant  of Latin-1 that replaces Icelandic letters
91              with Turkish ones.
92
93       8859-10 (Latin-6)
94              Latin-6 added the Inuit (Greenlandic) and Sami (Lappish) letters
95              that were missing in Latin-4 to cover the entire Nordic area.
96
97       8859-11
98              Supports  the  Thai  alphabet  and  is  nearly  identical to the
99              TIS-620 standard.
100
101       8859-12
102              This character set does not exist.
103
104       8859-13 (Latin-7)
105              Supports the Baltic Rim languages; in  particular,  it  includes
106              Latvian characters not found in Latin-4.
107
108       8859-14 (Latin-8)
109              This  is  the  Celtic  character  set, covering Old Irish, Manx,
110              Gaelic, Welsh, Cornish, and Breton.
111
112       8859-15 (Latin-9)
113              Latin-9 is similar to the widely used Latin-1 but replaces  some
114              less  common  symbols  with the Euro sign and French and Finnish
115              letters that were missing in Latin-1.
116
117       8859-16 (Latin-10)
118              This character set covers many Southeast European languages, and
119              most importantly supports Romanian more completely than Latin-2.
120
121   KOI8-R / KOI8-U
122       KOI8-R  is  a  non-ISO  character set popular in Russia before Unicode.
123       The lower half is ASCII; the upper is a Cyrillic character set somewhat
124       better  designed  than ISO 8859-5.  KOI8-U, based on KOI8-R, has better
125       support for Ukrainian.  Neither of these sets are ISO-2022  compatible,
126       unlike the ISO 8859 series.
127
128       Console  support  for KOI8-R is available under Linux through user-mode
129       utilities that modify keyboard bindings and the EGA graphics table, and
130       employ the "user mapping" font table in the console driver.
131
132   GB 2312
133       GB  2312  is a mainland Chinese national standard character set used to
134       express simplified Chinese.  Just  like  JIS  X  0208,  characters  are
135       mapped  into  a 94x94 two-byte matrix used to construct EUC-CN.  EUC-CN
136       is the most important encoding for Linux  and  includes  ASCII  and  GB
137       2312.  Note that EUC-CN is often called as GB, GB 2312, or CN-GB.
138
139   Big5
140       Big5  was a popular character set in Taiwan to express traditional Chi‐
141       nese.  (Big5 is both a character set and an encoding.)  It is a  super‐
142       set  of ASCII.  Non-ASCII characters are expressed in two bytes.  Bytes
143       0xa1–0xfe are used as leading bytes for two-byte characters.  Big5  and
144       its  extension were widely used in Taiwan and Hong Kong.  It is not ISO
145       2022 compliant.
146
147   JIS X 0208
148       JIS X 0208 is a Japanese national standard character set.  Though there
149       are  some  more  Japanese  national standard character sets (like JIS X
150       0201, JIS X 0212, and JIS X 0213), this  is  the  most  important  one.
151       Characters  are mapped into a 94x94 two-byte matrix, whose each byte is
152       in the range 0x21–0x7e.  Note that JIS X 0208 is a character  set,  not
153       an  encoding.   This  means  that JIS X 0208 itself is not used for ex‐
154       pressing text data.  JIS X 0208 is used as a component to construct en‐
155       codings such as EUC-JP, Shift_JIS, and ISO-2022-JP.  EUC-JP is the most
156       important encoding for Linux and includes ASCII and  JIS  X  0208.   In
157       EUC-JP, JIS X 0208 characters are expressed in two bytes, each of which
158       is the JIS X 0208 code plus 0x80.
159
160   KS X 1001
161       KS X 1001 is a Korean national standard character set.  Just as  JIS  X
162       0208, characters are mapped into a 94x94 two-byte matrix.  KS X 1001 is
163       used like JIS X 0208, as a component to  construct  encodings  such  as
164       EUC-KR,  Johab, and ISO-2022-KR.  EUC-KR is the most important encoding
165       for Linux and includes ASCII and KS X 1001.  KS C 5601 is an older name
166       for KS X 1001.
167
168   ISO 2022 and ISO 4873
169       The  ISO 2022 and 4873 standards describe a font-control model based on
170       VT100 practice.  This model is (partially) supported by the Linux  ker‐
171       nel  and  by xterm(1).  Several ISO 2022-based character encodings have
172       been defined, especially for Japanese.
173
174       There are 4 graphic character sets, called G0, G1, G2, and G3, and  one
175       of them is the current character set for codes with high bit zero (ini‐
176       tially G0), and one of them is the current character set for codes with
177       high  bit  one (initially G1).  Each graphic character set has 94 or 96
178       characters, and is essentially a 7-bit character set.   It  uses  codes
179       either  040–0177  (041–0176)  or  0240–0377 (0241–0376).  G0 always has
180       size 94 and uses codes 041–0176.
181
182       Switching between character sets is done using the shift  functions  ^N
183       (SO or LS1), ^O (SI or LS0), ESC n (LS2), ESC o (LS3), ESC N (SS2), ESC
184       O (SS3), ESC ~ (LS1R), ESC } (LS2R), ESC | (LS3R).   The  function  LSn
185       makes  character  set  Gn the current one for codes with high bit zero.
186       The function LSnR makes character set Gn the current one for codes with
187       high  bit  one.  The function SSn makes character set Gn (n=2 or 3) the
188       current one for the next character only (regardless of the value of its
189       high order bit).
190
191       A  94-character  set is designated as Gn character set by an escape se‐
192       quence ESC ( xx (for G0), ESC ) xx (for G1), ESC * xx (for G2),  ESC  +
193       xx (for G3), where xx is a symbol or a pair of symbols found in the ISO
194       2375 International Register of Coded Character Sets.  For example,  ESC
195       (  @  selects  the  ISO 646 character set as G0, ESC ( A selects the UK
196       standard character set (with pound instead of number sign), ESC ( B se‐
197       lects  ASCII  (with dollar instead of currency sign), ESC ( M selects a
198       character set for African languages, ESC ( ! A selects the Cuban  char‐
199       acter set, and so on.
200
201       A  96-character  set is designated as Gn character set by an escape se‐
202       quence ESC - xx (for G1), ESC . xx (for G2) or ESC / xx (for G3).   For
203       example, ESC - G selects the Hebrew alphabet as G1.
204
205       A  multibyte  character set is designated as Gn character set by an es‐
206       cape sequence ESC $ xx or ESC $ ( xx (for G0), ESC $ ) xx (for G1), ESC
207       $  *  xx (for G2), ESC $ + xx (for G3).  For example, ESC $ ( C selects
208       the Korean character set for G0.  The Japanese character  set  selected
209       by ESC $ B has a more recent version selected by ESC & @ ESC $ B.
210
211       ISO 4873 stipulates a narrower use of character sets, where G0 is fixed
212       (always ASCII), so that G1, G2, and G3 can be invoked  only  for  codes
213       with  the  high  order  bit set.  In particular, ^N and ^O are not used
214       anymore, ESC ( xx can be used only with xx=B, and ESC ) xx, ESC  *  xx,
215       ESC + xx are equivalent to ESC - xx, ESC . xx, ESC / xx, respectively.
216
217   TIS-620
218       TIS-620  is  a  Thai  national standard character set and a superset of
219       ASCII.  In the same fashion as the ISO 8859 series, Thai characters are
220       mapped into 0xa1–0xfe.
221
222   Unicode
223       Unicode  (ISO/IEC 10646) is a standard which aims to unambiguously rep‐
224       resent every character in every human  language.   Unicode's  structure
225       permits  20.1  bits  to  encode  every character.  Since most computers
226       don't include 20.1-bit integers, Unicode is usually encoded  as  32-bit
227       integers  internally  and  either  a series of 16-bit integers (UTF-16)
228       (needing two 16-bit integers only when encoding  certain  rare  charac‐
229       ters) or a series of 8-bit bytes (UTF-8).
230
231       Linux  represents Unicode using the 8-bit Unicode Transformation Format
232       (UTF-8).  UTF-8 is a variable length encoding of Unicode.   It  uses  1
233       byte  to code 7 bits, 2 bytes for 11 bits, 3 bytes for 16 bits, 4 bytes
234       for 21 bits, 5 bytes for 26 bits, 6 bytes for 31 bits.
235
236       Let 0,1,x stand for a zero, one, or arbitrary  bit.   A  byte  0xxxxxxx
237       stands for the Unicode 00000000 0xxxxxxx which codes the same symbol as
238       the ASCII 0xxxxxxx.  Thus, ASCII goes unchanged into UTF-8, and  people
239       using only ASCII do not notice any change: not in code, and not in file
240       size.
241
242       A byte 110xxxxx is the start of a 2-byte code, and 110xxxxx 10yyyyyy is
243       assembled  into  00000xxx  xxyyyyyy.  A byte 1110xxxx is the start of a
244       3-byte code, and 1110xxxx 10yyyyyy 10zzzzzz is assembled into  xxxxyyyy
245       yyzzzzzz.   (When  UTF-8  is used to code the 31-bit ISO/IEC 10646 then
246       this progression continues up to 6-byte codes.)
247
248       For most texts in ISO 8859 character sets, this means that the  charac‐
249       ters  outside of ASCII are now coded with two bytes.  This tends to ex‐
250       pand ordinary text files by only one or two percent.   For  Russian  or
251       Greek  texts,  this  expands ordinary text files by 100%, since text in
252       those languages is mostly outside of ASCII.  For  Japanese  users  this
253       means  that  the  16-bit codes now in common use will take three bytes.
254       While there are algorithmic conversions from some character sets (espe‐
255       cially  ISO  8859-1)  to  Unicode, general conversion requires carrying
256       around conversion tables, which can be quite large for 16-bit codes.
257
258       Note that UTF-8 is self-synchronizing: 10xxxxxx is a  tail,  any  other
259       byte  is  the head of a code.  Note that the only way ASCII bytes occur
260       in a UTF-8 stream, is as themselves.  In particular, there are  no  em‐
261       bedded NULs ('\0') or '/'s that form part of some larger code.
262
263       Since ASCII, and, in particular, NUL and '/', are unchanged, the kernel
264       does not notice that UTF-8 is being used.  It does not care at all what
265       the bytes it is handling stand for.
266
267       Rendering  of  Unicode  data streams is typically handled through "sub‐
268       font" tables which map a subset of Unicode to glyphs.   Internally  the
269       kernel  uses Unicode to describe the subfont loaded in video RAM.  This
270       means that in the Linux console in UTF-8 mode, one can use a  character
271       set  with 512 different symbols.  This is not enough for Japanese, Chi‐
272       nese, and Korean, but it is enough for most other purposes.
273

SEE ALSO

275       iconv(1), ascii(7), iso_8859-1(7), unicode(7), utf-8(7)
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278
279Linux man-pages 6.05              2023-03-12                       charsets(7)
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