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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 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,  Portuguese,
              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 Taiwan 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  construct  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 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 sequence 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 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)

COLOPHON

       This page is part of release 4.13 of the Linux man-pages project.  A  description  of  the
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