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PRECONV(1) General Commands Manual PRECONV(1)
NAME
preconv - convert encoding of input files to something GNU troff understands
SYNOPSIS
preconv [-dr] [-D default_encoding] [-e encoding] [file ...]
preconv -h
preconv --help
preconv -v
preconv --version
DESCRIPTION
preconv reads files and converts its encoding(s) to a form GNU troff(1) can process, send-
ing the data to standard output. Currently, this means ASCII characters and '\[uXXXX]'
entities, where 'XXXX' is a hexadecimal number with four to six digits, representing a
Unicode input code. Normally, preconv should be invoked with the -k and -K options of
groff.
OPTIONS
Whitespace is permitted between a command-line option and its argument.
-d Emit debugging messages to standard error (mainly the used encoding).
-Dencoding
Specify default encoding if everything fails (see below).
-eencoding
Specify input encoding explicitly, overriding all other methods. This corresponds
to groff's -Kencoding option. Without this switch, preconv uses the algorithm de-
scribed below to select the input encoding.
--help
-h Print a help message and exit.
-r Do not add .lf requests.
--version
-v Print the version number and exit.
USAGE
preconv tries to find the input encoding with the following algorithm.
1. If the input encoding has been explicitly specified with option -e, use it.
2. Otherwise, check whether the input starts with a Byte Order Mark (BOM, see below).
If found, use it.
3. Otherwise, check whether there is a known coding tag (see below) in either the
first or second input line. If found, use it.
4 Finally, if the uchardet library (an encoding detector library available on most
major distributions) is available on the system, use it to try to detect the encod-
ing of the file.
5. If everything fails, use a default encoding as given with option -D, by the current
locale, or 'latin1' if the locale is set to 'C', 'POSIX', or empty (in that order).
Note that the groff program supports a GROFF_ENCODING environment variable which is even-
tually expanded to option -k.
Byte Order Mark
The Unicode Standard defines character U+FEFF as the Byte Order Mark (BOM). On the other
hand, value U+FFFE is guaranteed not be a Unicode character at all. This allows detection
of the byte order within the data stream (either big-endian or little-endian), and the
MIME encodings 'UTF-16' and 'UTF-32' mandate that the data stream starts with U+FEFF.
Similarly, the data stream encoded as 'UTF-8' might start with a BOM (to ease the conver-
sion from and to UTF-16 and UTF-32). In all cases, the byte order mark is not part of the
data but part of the encoding protocol; in other words, preconv's output doesn't contain
it.
Note that U+FEFF not at the start of the input data actually is emitted; it has then the
meaning of a 'zero width no-break space' character - something not needed normally in
groff.
Coding Tags
Editors which 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 heuristic algorithms for data which represents a greater amount of a nat-
ural language, it is still just a guess. Additionally, all algorithms fail easily for in-
put which is either too short or doesn't represent a natural language.
For these reasons, preconv supports the coding tag convention (with some restrictions) as
used by GNU Emacs and XEmacs (and probably other programs too).
Coding tags in GNU Emacs and XEmacs are stored in so-called File Variables. preconv rec-
ognizes the following syntax form which must be put into a troff comment in the first or
second line.
-*- tag1: value1; tag2: value2; ... -*-
The only relevant tag for preconv is 'coding' which can take the values listed below.
Here an example line which tells Emacs to edit a file in troff mode, and to use latin2 as
its encoding.
.\" -*- mode: troff; coding: latin-2 -*-
The following list gives all MIME coding tags (either lowercase or uppercase) supported by
preconv; this list is hard-coded in the source.
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 hard-coded list of other tags is recognized which eventually
map to values 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
Those tags are taken from GNU Emacs and XEmacs, together with some aliases. Trailing
'-dos', '-unix', and '-mac' suffixes of coding tags (which give the end-of-line convention
used in the file) are stripped off before the comparison with the above tags happens.
Iconv Issues
preconv by itself only supports three encodings: latin-1, cp1047, and UTF-8; all other en-
codings are passed to the iconv library functions. At compile time it is searched and
checked for a valid iconv implementation; a call to 'preconv --version' shows whether
iconv is used.
BUGS
preconv doesn't support local variable lists yet. This is a different syntax form to
specify local variables at the end of a file.
SEE ALSO
groff(1)
the GNU Emacs and XEmacs info pages
groff 1.22.4 23 March 2022 PRECONV(1)
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