man > charsets(7)

charsets(7)                       Miscellaneous Information 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 information  is  still  helpful
       for people working with legacy systems and documents.

       Standards  discussed  include such as ASCII, GB 2312, ISO/IEC 8859, JIS, KOI8-R, KS, and Uni‐
       code.

       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  de‐
       scribed by the ISO/IEC 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 oth‐
       ers 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/IEC 8859
       ISO/IEC  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 graph‐
       ics in positions 160–255.

       Of these, the most important is ISO/IEC 8859-1 ("Latin Alphabet No. 1" /  Latin-1).   It  was
       widely  adopted and supported by different systems, and is gradually being replaced with Uni‐
       code.  The ISO/IEC 8859-1 characters are also the first 256 characters of Unicode.

       Console support for the other ISO/IEC 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 character set:

       ISO/IEC 8859-1 (Latin-1)
              Latin-1 covers many 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 „German“ quotation  marks
              was considered tolerable.

       ISO/IEC 8859-2 (Latin-2)
              Latin-2  supports many Latin-written Central and East European languages such as Bosn‐
              ian, Croatian, Czech, German, Hungarian, Polish, Slovak, and Slovene.   Replacing  Ro‐
              manian ș/ț with ş/ţ was considered tolerable.

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              later superseded it for Turkish.

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              Lithuanian, but was superseded by ISO/IEC 8859-10 and ISO/IEC 8859-13.

       ISO/IEC 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.

       ISO/IEC 8859-6
              Was  created  for  Arabic.  The ISO/IEC 8859-6 glyph table is a fixed font of separate
              letter forms, but a proper display engine should combine these using the  proper  ini‐
              tial, medial, and final forms.

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

       ISO/IEC 8859-8
              Supports Modern Hebrew without niqud (punctuation signs).  Niqud and full-fledged Bib‐
              lical Hebrew were outside the scope of this character set.

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

       ISO/IEC 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.

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

       ISO/IEC 8859-12
              This character set does not exist.

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

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

       ISO/IEC 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.

       ISO/IEC 8859-16 (Latin-10)
              This character set covers many Southeast European languages, and most importantly sup‐
              ports 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/IEC 8859-5.  KOI8-U,
       based on KOI8-R, has better support for Ukrainian.  Neither of these sets  are  ISO/IEC  2022
       compatible, unlike the ISO/IEC 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 Chi‐
       nese.  Just like JIS X 0208, characters are mapped into a 94x94 two-byte matrix used to  con‐
       struct  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 ex‐
       pressed 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/IEC 2022 com‐
       pliant.

   JIS X 0208
       JIS  X 0208 is a Japanese national standard character set.  Though there are some more Japan‐
       ese 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/IEC 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/IEC 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/IEC 2022 and ISO/IEC 4873
       The  ISO/IEC  2022  and  ISO/IEC  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/IEC 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/IEC 2375 International Register of Coded Character Sets.  For  exam‐
       ple,  ESC  (  @  selects the ISO/IEC 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  ex‐
       ample,  ESC  $  (  C selects the Korean character set for G0.  The Japanese character set se‐
       lected by ESC $ B has a more recent version selected by ESC & @ ESC $ B.

       ISO/IEC 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 partic‐
       ular,  ^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 fash‐
       ion as the ISO/IEC 8859 series, Thai characters are mapped into 0xa1–0xfe.

   Unicode
       Unicode (ISO/IEC 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 inte‐
       gers 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  un‐
       changed  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/IEC
       10646 then this progression continues up to 6-byte codes.)

       For most texts in ISO/IEC 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  conver‐
       sions from some character sets (especially ISO/IEC 8859-1) to Unicode, general conversion re‐
       quires 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 par‐
       ticular, 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 char‐
       acter  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 man-pages 6.7                          2024-01-28                                  charsets(7)