Section: User Commands (1)
Updated: 17 March 2021
preconv - convert encoding of input files to something GNU troff understands
[ -D default_encoding
[ -e encoding
and converts its encoding(s) to a form GNU
can process, sending 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.
should be invoked with the
Whitespace is permitted between a command-line option and its argument.
Emit debugging messages to standard error (mainly the used encoding).
Specify default encoding if everything fails (see below).
Specify input encoding explicitly, overriding all other methods.
This corresponds to
Without this switch,
uses the algorithm described below to select the input encoding.
Print a help message and exit.
Do not add .lf requests.
Print the version number and exit.
tries to find the input encoding with the following algorithm.
If the input encoding has been explicitly specified with option
Otherwise, check whether the input starts with a
Byte Order Mark
(BOM, see below).
If found, use it.
Otherwise, check whether there is a known
(see below) in either the first or second input line.
If found, use it.
Finally, if the
(an encoding detector library available on most major distributions)
is available on the system, use it to try to detect the encoding of the file.
If everything fails, use a default encoding as given with option
by the current locale, or 'latin1' if the locale is set to
'C', 'POSIX', or empty (in that order).
Note that the
program supports a
environment variable which is eventually expanded to option
Byte Order Mark
The Unicode Standard defines character U+FEFF as the Byte Order Mark
On the other hand, value U+FFFE is guaranteed not be a Unicode character at
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 conversion from and to UTF-16 and UTF-32).
In all cases, the byte order mark is
part of the data but part of the encoding protocol; in other words,
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
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 natural
language, it is still just a guess.
Additionally, all algorithms fail easily for input which is either too short
or doesn't represent a natural language.
For these reasons,
supports the coding tag convention (with some restrictions) as used by
(and probably other programs too).
Coding tags in
are stored in so-called
recognizes the following syntax form which must be put into a troff comment
in the first or second line.
The only relevant tag for
is 'coding' which can take the values listed below.
Here an example line which tells
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
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,
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,
Those tags are taken from
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
by itself only supports three encodings: latin-1, cp1047, and UTF-8;
all other encodings are passed to the
At compile time it is searched and checked for a valid
implementation; a call to 'preconv --version' shows whether
local variable lists
This is a different syntax form to specify local variables at the end of a