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Name | Synopsis | Description | Options | See Also | COLOPHON |
preconv(1) General Commands Manual preconv(1)
preconv - prepare files for typesetting with GNU roff
preconv [-dr] [-D default-encoding] [-e encoding] [file ...]
preconv -h
preconv --help
preconv -v
preconv --version
preconv reads each file, converts its encoded characters to a form
groff(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 behavior.
In typical usage scenarios, preconv need not be run directly; instead
it should be invoked with the -k or -K options of groff.
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).
Note that 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.
Furthermore, groff supports a GROFF_ENCODING environment variable
which is equivalent to its option -k.
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 non-breaking
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 rec‐
ognized, 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
preconv itself only supports 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 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). Com‐
pare 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 sys‐
tem may behave differently) and preconv falls back to the locale set‐
tings, 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.
-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.
groff(1), iconv(3), locale(7)
This page is part of the groff (GNU troff) project. Information
about the project can be found at
⟨http://www.gnu.org/software/groff/⟩. If you have a bug report for
this manual page, see ⟨http://www.gnu.org/software/groff/⟩. This
page was obtained from the project's upstream Git repository
⟨https://git.savannah.gnu.org/git/groff.git⟩ on 2020-08-13. (At that
time, the date of the most recent commit that was found in the repos‐
itory was 2020-08-12.) If you discover any rendering problems in
this HTML version of the page, or you believe there is a better or
more up-to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not part
of the original manual page), send a mail to man-pages@man7.org
groff 1.22.4.234-3ba6 12 August 2020 preconv(1)
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