PRECONV(1) General Commands Manual PRECONV(1)
preconv - convert encoding of input files to something GNU troff under-
preconv [-dr] [-e encoding] [files ...]
preconv -h | --help
preconv -v | --version
It is possible to have whitespace between the -e command line option
and its parameter.
preconv reads files and converts its encoding(s) to a form GNU troff(1)
can process, sending the data to standard output. Currently, this
means ASCII characters and '\[uXXXX]' entities, where 'XXXX' is a hexa-
decimal number with four to six digits, representing a Unicode input
code. Normally, preconv should be invoked with the -k and -K options
-d Emit debugging messages to standard error (mainly the used
Specify default encoding if everything fails (see below).
Specify input encoding explicitly, overriding all other methods.
This corresponds to groff's -Kencoding option. Without this
switch, preconv uses the algorithm described below to select the
-h Print help message.
-r Do not add .lf requests.
-v Print version number.
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. Finally, check whether there is a known coding tag (see below)
in either the first or second input line. If found, use it.
4. 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 vari-
able which is eventually 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 to detect the byte order within the data
stream (either big-endian or lower-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 conversion 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 emit-
ted; it has then the meaning of a 'zero width no-break space' character
- something not needed normally in groff.
Editors which support more than a single character encoding need tags
within the input files to mark the file's encoding. While it is possi-
ble to guess the right input encoding with the help of heuristic algo-
rithms for data which represents a greater amount of a natural lan-
guage, it is still just a guess. Additionally, all algorithms fail
easily for input which is either too short or doesn't represent a natu-
For these reasons, preconv supports the coding tag convention (with
some restrictions) as used by GNU Emacs and XEmacs (and probably other
Coding tags in GNU Emacs and XEmacs are stored in so-called File Vari-
ables. preconv recognizes 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,
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.
preconv by itself only supports three encodings: latin-1, cp1047, and
UTF-8; all other encodings are passed to the iconv library functions.
At compile time it is searched and checked for a valid iconv implemen-
tation; a call to 'preconv --version' shows whether iconv is used.
preconv doesn't support local variable lists yet. This is a different
syntax form to specify local variables at the end of a file.
the GNU Emacs and XEmacs info pages
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Groff Version 1.22.3 10 February 2018 PRECONV(1)
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