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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 (noncombining) 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.org>.

       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 3.35 of the Linux man-pages project.  A  description  of  the
       project,  and information about reporting bugs, can be found at http://man7.org/linux/man-
       pages/.