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NAME

       charsets - programmer’s view of character sets and internationalization

DESCRIPTION

       Linux is an international operating system.  Various of  its  utilities
       and  device drivers (including the console driver) support multilingual
       character sets including Latin-alphabet letters with diacritical marks,
       accents,  ligatures,  and  entire  non-Latin alphabets including Greek,
       Cyrillic, Arabic, and Hebrew.

       This  manual  page  presents  a  programmer’s-eye  view  of   different
       character-set  standards and how they fit together on Linux.  Standards
       discussed include ASCII, ISO 8859, KOI8-R, Unicode, ISO  2022  and  ISO
       4873.   The  primary  emphasis  is  on  character sets actually used as
       locale character sets, not the myriad others that can be found in  data
       from other systems.

       A  complete  list of charsets used in an officially supported locale in
       glibc  2.2.3  is:  ISO-8859-{1,2,3,5,6,7,8,9,13,15},   CP1251,   UTF-8,
       EUC-{KR,JP,TW},  KOI8-{R,U}, GB2312, GB18030, GBK, BIG5, BIG5-HKSCS and
       TIS-620 (in no  particular  order.)   (Romanian  may  be  switching  to
       ISO-8859-16.)

   ASCII
       ASCII  (American  Standard  Code  For  Information  Interchange) is the
       original 7-bit character set, originally designed for American English.
       It is currently described by the ECMA-6 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 exist.
       All are deprecated; glibc doesn’t support locales whose character  sets
       aren’t  true supersets of ASCII. (These sets are also known as ISO-646,
       a close relative of ASCII that permitted replacing these characters.)

       As Linux was written for hardware  designed  in  the  US,  it  natively
       supports ASCII.

   ISO 8859
       ISO  8859  is  a series of 15 8-bit character sets all of which have US
       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-1).  It  is  natively
       supported  in the Linux console driver, fairly well supported in X11R6,
       and is the base character set of HTML.

       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 most Western European languages such as Albanian,
              Catalan,  Danish,  Dutch,  English,  Faroese,  Finnish,  French,
              German,   Galician,   Irish,   Icelandic,   Italian,  Norwegian,
              Portuguese, Spanish, and Swedish.  The  lack  of  the  ligatures
              Dutch  ij, French oe and old-style ,,German‘‘ quotation marks is
              considered tolerable.

       8859-2 (Latin-2)
              Latin-2 supports most Latin-written Slavic and Central  European
              languages: Croatian, Czech, German, Hungarian, Polish, Rumanian,
              Slovak, and Slovene.

       8859-3 (Latin-3)
              Latin-3 is popular with  authors  of  Esperanto,  Galician,  and
              Maltese.  (Turkish is now written with 8859-9 instead.)

       8859-4 (Latin-4)
              Latin-4   introduced   letters   for   Estonian,   Latvian,  and
              Lithuanian.  It is essentially obsolete; see  8859-10  (Latin-6)
              and 8859-13 (Latin-7).

       8859-5 Cyrillic letters supporting Bulgarian, Byelorussian, Macedonian,
              Russian, Serbian and  Ukrainian.   Ukrainians  read  the  letter
              ‘ghe’  with  downstroke  as  ‘heh’  and  would  need  a ghe with
              upstroke to write a correct ghe.  See the discussion  of  KOI8-R
              below.

       8859-6 Supports  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 Supports Modern Greek.

       8859-8 Supports modern Hebrew without niqud (punctuation signs).  Niqud
              and  full-fledged  Biblical Hebrew are outside the scope of this
              character set; under Linux, UTF-8 is the preferred encoding  for
              these.

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

       8859-10 (Latin-6)
              Latin 6 adds the last Inuit  (Greenlandic)  and  Sami  (Lappish)
              letters  that were missing in Latin 4 to cover the entire Nordic
              area.  RFC 1345 listed a  preliminary  and  different  ‘latin6’.
              Skolt Sami still needs a few more accents than these.

       8859-11
              This  only  exists  as  a  rejected  draft  standard.  The draft
              standard was identical to TIS-620, which is used under Linux for
              Thai.

       8859-12
              This  set  does  not exist.  While Vietnamese has been suggested
              for this space, it does not fit within  the  96  (non-combining)
              characters  ISO  8859  offers.  UTF-8 is the preferred character
              set for Vietnamese use under Linux.

       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 Gaelic and Welsh.
              This charset also contains the dotted characters needed for  Old
              Irish.

       8859-15 (Latin-9)
              This adds the Euro sign and French and Finnish letters that were
              missing in Latin-1.

       8859-16 (Latin-10)
              This set covers many of the languages  covered  by  8859-2,  and
              supports Romanian more completely then that set does.

   KOI8-R
       KOI8-R is a non-ISO character set popular in Russia.  The lower half is
       US ASCII; the  upper  is  a  Cyrillic  character  set  somewhat  better
       designed  than ISO 8859-5.  KOI8-U is a common character set, based off
       KOI8-R, that 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.

   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 US 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 US ASCII and KS X 1001.  KS C 5601 is an older
       name for KS X 1001.

   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 US ASCII and GB
       2312.  Note that EUC-CN is often called as GB, GB 2312, or CN-GB.

   Big5
       Big5 is a popular  character  set  in  Taiwan  to  express  traditional
       Chinese.   (Big5  is  both  a  character set and an encoding.)  It is a
       superset of US 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 is widely used in Taiwan and Hong Kong.   It  is
       not ISO 2022-compliant.

   TIS 620
       TIS  620 is a Thai national standard character set and a superset of US
       ASCII.   Like  ISO  8859  series,  Thai  characters  are  mapped   into
       0xa1-0xfe.  TIS 620 is the only commonly used character set under Linux
       besides UTF-8 to have combining characters.

   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).  Information on Unicode
       is available at <http://www.unicode.com>.

       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 people who use 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 users, 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  (esp.
       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 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.

       At the current time, the  console  driver  does  not  handle  combining
       characters.   So  Thai,  Sioux  and  any other script needing combining
       characters can’t be handled on the console.

   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).  It is popular in Japan and Korea.

       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, etc. etc.

       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 only be invoked 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.

SEE ALSO

       console(4),      console_codes(4),      console_ioctl(4),     ascii(7),
       iso_8859-1(7), unicode(7), utf-8(7)

COLOPHON

       This page is part of release 2.77 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.