charsets

CHARSETS(7)                 Linux Programmer's 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 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 5.08 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.



Linux                              2020-08-13                        CHARSETS(7)