preconv(1) — Linux manual page

Name | Synopsis | Description | Options | See also | COLOPHON

preconv(1)               General Commands Manual              preconv(1)

Name         top

       preconv - prepare files for typesetting with groff

Synopsis         top

       preconv [-dr] [-D default-encoding] [-e encoding] [file ...]

       preconv -h
       preconv --help

       preconv -v
       preconv --version

Description         top

       preconv reads each file, converts its encoded characters to a
       form troff(1) can interpret, and sends the result to the standard
       output stream.  Currently, this means that code points in the
       range 0–127 (in US-ASCII, ISO 8859, or Unicode) remain as-is and
       the remainder are converted to the groff special character form
       “\[uXXXX]”, where XXXX is a hexadecimal number of four to six
       digits corresponding to a Unicode code point.  By default,
       preconv also inserts a roff .lf request at the beginning of each
       file, identifying it for the benefit of later processing
       (including diagnostic messages); the -r option suppresses this

       In typical usage scenarios, preconv need not be run directly;
       instead it should be invoked with the -k or -K options of groff.
       If preconv is given no file operands, it reads the standard input

       preconv tries to find the input encoding with the following
       algorithm, stopping at the first success.

       1.     If the input encoding has been explicitly specified with
              option -e, use it.

       2.     Check whether the input starts with a Unicode Byte Order
              Mark.  If so, determine the encoding as UTF-8, UTF-16, or
              UTF-32 accordingly.

       3.     If the input stream is seekable, check the first and
              second input lines for a recognized GNU Emacs file-local
              variable identifying the character encoding, here referred
              to as the “coding tag” for brevity.  If found, use it.

       4.     If the input stream is seekable, and if the uchardet
              library is available on the system, use it to try to infer
              the encoding of the file.

       5.     If the -D option specifies an encoding, use it.

       6.     Use the encoding specified by the current locale
              (LC_CTYPE), unless the locale is “C”, “POSIX”, or empty,
              in which case assume Latin-1 (ISO 8859-1).

       The coding tag and uchardet methods in the above procedure rely
       upon a seekable input stream; when preconv reads from a pipe, the
       stream is not seekable, and these detection methods are skipped.
       If character encoding detection of your input files is
       unreliable, arrange for one of the other methods to succeed by
       using preconv's -D or -e options, or by configuring your locale
       appropriately.  groff also supports a GROFF_ENCODING environment
       variable, which can be overridden by its -K option.  Valid values
       for (or parameters to) all of these are enumerated in the lists
       of recognized coding tags in the next subsection, and is further
       influenced by iconv library support.

   Coding tags
       Text editors that support more than a single character encoding
       need tags within the input files to mark the file's encoding.
       While it is possible to guess the right input encoding with the
       help of heuristics that are reliable for a preponderance of
       natural language texts, they are not absolutely reliable.
       Heuristics can fail on inputs that are too short or don't
       represent a natural language.

       Consequently, preconv supports the coding tag convention (with
       some restrictions) used by GNU Emacs.  These are indicated in
       specially marked regions of an input file designated for “file-
       local variables”.

       preconv interprets the following syntax if it occurs in a roff
       comment in the first or second line of the input file.  Both “\"”
       and “\#” comment forms are recognized, but the control (or no-
       break control) character must be the default and must begin the
       line.  Similarly, the escape character must be the default.
              -*- [...;] coding: encoding[; ...] -*-

       The only variable preconv interprets is “coding”, which can take
       the values listed below.

       The following list comprises all MIME “charset” parameter values
       recognized, case-insensitively, by preconv.
              big5, cp1047, euc-jp, euc-kr, gb2312, iso-8859-1,
              iso-8859-2, iso-8859-5, iso-8859-7, iso-8859-9,
              iso-8859-13, iso-8859-15, koi8-r, us-ascii, utf-8, utf-16,
              utf-16be, utf-16le

       In addition, the following list of other coding tags is
       recognized, each of which is mapped to an appropriate value from
       the list above.
              ascii, chinese-big5, chinese-euc, chinese-iso-8bit,
              cn-big5, cn-gb, cn-gb-2312, cp878, csascii, csisolatin1,
              cyrillic-iso-8bit, cyrillic-koi8, euc-china, euc-cn,
              euc-japan, euc-japan-1990, euc-korea, greek-iso-8bit,
              iso-10646/utf8, iso-10646/utf-8, iso-latin-1, iso-latin-2,
              iso-latin-5, iso-latin-7, iso-latin-9, japanese-euc,
              japanese-iso-8bit, jis8, koi8, korean-euc,
              korean-iso-8bit, latin-0, latin1, latin-1, latin-2,
              latin-5, latin-7, latin-9, mule-utf-8, mule-utf-16,
              mule-utf-16be, mule-utf-16-be,
              mule-utf-16be-with-signature, mule-utf-16le,
              mule-utf-16-le, mule-utf-16le-with-signature, utf8,
              utf-16-be, utf-16-be-with-signature,
              utf-16be-with-signature, utf-16-le,
              utf-16-le-with-signature, utf-16le-with-signature

       Trailing “-dos”, “-unix”, and “-mac” suffixes on coding tags
       (which indicate the end-of-line convention used in the file) are
       disregarded for the purpose of comparison with the above tags.

   iconv support
       While preconv recognizes all of the coding tags listed above, it
       is capable on its own of interpreting only three encodings:
       Latin-1, code page 1047, and UTF-8.  If iconv support is
       configured at compile time and available at run time, all other
       encodings are passed to iconv library functions.  The library may
       recognize many additional encoding strings.  The command
       “preconv -v” discloses whether iconv support is configured.

       The use of iconv means that characters in the input that encode
       invalid code points for that encoding may be dropped from the
       output stream or mapped to the Unicode replacement character
       (U+FFFD).  Compare the following examples using the input “café”
       (note the “e” with an acute accent), which due to its short
       length challenges inference of the encoding used.
              printf 'caf\351\n' | LC_ALL=en_US.UTF-8 preconv
              printf 'caf\351\n' | preconv -e us-ascii
              printf 'caf\351\n' | preconv -e latin-1
       The fate of the accented “e” differs in each case.  In the first,
       uchardet fails to detect an encoding (though the library on your
       system may behave differently) and preconv falls back to the
       locale settings, where octal 351 starts an incomplete UTF-8
       sequence and results in the Unicode replacement character.  In
       the second, it is not a representable character in the declared
       input encoding of US-ASCII and is discarded by iconv.  In the
       last, it is correctly detected and mapped.

Options         top

       -h and --help display a usage message, while -v and --version
       show version information; all exit afterward.

       -d     Emit debugging messages to the standard error stream.

       -D default-encoding
              Report default-encoding if all detection methods fail.

       -e encoding
              Override detection procedure and assume encoding.  This
              corresponds to groff's “-K encoding” option.

       -r     Write files “raw”; do not add .lf requests.

See also         top

       groff(1), iconv(3), locale(7)

COLOPHON         top

       This page is part of the groff (GNU troff) project.  Information
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       for this manual page, see ⟨⟩.
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groff 1.23.0.rc1.654-4e1db-dir1t9yAugust 2021                   preconv(1)

Pages that refer to this page: groff(1)