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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 a 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; GNU libc 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 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)