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CHARSETS(7)                                                             Linux Programmer's 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 informa‐
       tion is still helpful for people working with legacy systems and documents.

       Standards discussed include such as ASCII, GB 2312, ISO 8859, JIS, KOI8-R, KS, and Unicode.

       The primary emphasis is on character sets that were actually used by locale 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 original 7-bit character set, originally designed for American  English.   Also  known  as  US-
       ASCII.  It is currently described by the ISO 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 8859
       ISO 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 positions 128 to 159, and  96
       fixed-width graphics in positions 160–255.

       Of  these, the most important is ISO 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 8859-1 characters are also the first 256 characters of Unicode.

       Console support for the other 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 mapping" font table in the console driver.

       Here are brief descriptions of each set:

       8859-1 (Latin-1)
              Latin-1  covers  many  West  European  languages  such as Albanian, Basque, Danish, English, Faroese, Galician, Icelandic, Irish, Italian, Norwegian, Por‐
              tuguese, Spanish, and Swedish.  The lack of the ligatures Dutch IJ/ij, French œ, and old-style „German“ quotation marks was considered tolerable.

       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.

       8859-3 (Latin-3)
              Latin-3 was designed to cover of Esperanto, Maltese, and Turkish, but 8859-9 later superseded it for Turkish.

       8859-4 (Latin-4)
              Latin-4 introduced letters for North European languages such as Estonian, Latvian, and Lithuanian, but was superseded by 8859-10 and 8859-13.

       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.

       8859-6 Was created for Arabic.  The 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.

       8859-7 Was created for Modern Greek in 1987, updated in 2003.

       8859-8 Supports Modern Hebrew without niqud (punctuation signs).  Niqud and full-fledged Biblical Hebrew were outside the scope of this character set.

       8859-9 (Latin-5)
              This is a variant of Latin-1 that replaces Icelandic letters with Turkish ones.

       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.

       8859-11
              Supports the Thai alphabet and is nearly identical to the TIS-620 standard.

       8859-12
              This set does not exist.

       8859-13 (Latin-7)
              Supports the Baltic Rim languages; in particular, it includes Latvian characters not found in Latin-4.

       8859-14 (Latin-8)
              This is the Celtic character set, covering Old Irish, Manx, Gaelic, Welsh, Cornish, and Breton.

       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.

       8859-16 (Latin-10)
              This 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 better designed than
       ISO 8859-5.  KOI8-U, based on KOI8-R, has better support for Ukrainian.  Neither of these sets are ISO-2022 compatible, unlike the ISO 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 Chinese.  (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 Tai‐
       wan and Hong Kong.  It is not ISO 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  con‐
       struct  encodings such as EUC-JP, Shift_JIS, and ISO-2022-JP.  EUC-JP is the most important 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-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 2022 and ISO 4873
       The ISO 2022 and 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 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 (initially 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 either 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 sequence 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 2375 International Register of Coded Character Sets.  For example, ESC ( @ selects the ISO 646 character set as
       G0, ESC ( A selects the UK standard character set (with pound instead of number sign), ESC ( B selects ASCII (with dollar instead of currency sign), ESC ( M  se‐
       lects 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 sequence ESC - xx (for G1), ESC . xx (for G2) or ESC / xx (for G3).  For example, ESC - G se‐
       lects 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 se‐
       lected by ESC & @ ESC $ B.

       ISO 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, respectively.

   TIS-620
       TIS-620 is a Thai national standard character set and a superset of ASCII.  In the same fashion  as  the  ISO  8859  series,  Thai  characters  are  mapped  into
       0xa1–0xfe.

   Unicode
       Unicode (ISO 10646) is a standard which aims to unambiguously represent every character in every human language.  Unicode's structure permits 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 10646 then this progression continues  up  to  6-byte
       codes.)

       For  most  texts  in  ISO  8859 character sets, this means that the characters outside of ASCII are now coded with two bytes.  This tends to expand 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 (especially ISO 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                                                                          2020-08-13                                                                    CHARSETS(7)