preconv(1) General Commands Manual preconv(1)
Name
preconv - prepare files for typesetting with groff
Synopsis
preconv [-dr] [-D fallback-encoding] [-e encoding] [file ...]
preconv -h
preconv --help
preconv -v
preconv --version
Description
preconv reads each file, converts its encoded characters to a form
]8;;man:troff(1)\troff(1)]8;;\ 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 con-
verted 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 pro-
cessing (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. If no file
operands are given on the command line, or if file is “-”, the standard
input stream is read.
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. If the input starts with a Unicode Byte Order Mark, 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 enumer-
ated in the lists of recognized coding tags in the next subsection, and
are 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 used by
GNU Emacs (with some restrictions). This notation appears 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 “\#” com-
ment forms are recognized, but the control (or no-break control) char-
acter must be the default and must begin the line. Similarly, the es-
cape 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 recog-
nized, 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 in-
dicate 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 ca-
pable 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 others are passed to iconv library func-
tions, which may recognize many additional encoding strings. The com-
mand “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 fol-
lowing examples using the input “café” (note the “e” with an acute ac-
cent), which due to its short length challenges inference of the encod-
ing 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 repre-
sentable character in the declared input encoding of US-ASCII and is
discarded by iconv. In the last, it is correctly detected and mapped.
Limitations
preconv cannot perform any transformation on input that it cannot see.
Examples include files that are interpolated by preprocessors that run
subsequently, including ]8;;man:soelim(1)\soelim(1)]8;;\; files included by troff itself
through “so” and similar requests; and string definitions passed to
troff through its -d command-line option.
preconv assumes that its input uses the default escape character, a
backslash \, and writes special character escape sequences accordingly.
Options
-h and --help display a usage message, while -v and --version show ver-
sion information; all exit afterward.
-d Emit debugging messages to the standard error stream.
-D fallback-encoding
Report fallback-encoding if all detection methods fail.
-e encoding
Skip detection and assume encoding; see groff's -K option.
-r Write files “raw”; do not add .lf requests.
See also
]8;;man:groff(1)\groff(1)]8;;\, ]8;;man:iconv(3)\iconv(3)]8;;\, ]8;;man:locale(7)\locale(7)]8;;\
groff 1.23.0 31 March 2024 preconv(1)
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