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preconv(1) General Commands Manual preconv(1)
preconv - prepare files for typesetting with groff
preconv [-dr] [-D fallback-encoding] [-e encoding] [file ...]
preconv -h
preconv --help
preconv -v
preconv --version
preconv reads each file, converts its encoded characters to a form
troff(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.
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 enumerated 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 “\#” comment forms are recognized, but the control (or no-
break 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
recognized, 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.
[1miconv[24m support
While preconv recognizes all of the coding tags listed above, it
is capable 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 functions, which may recognize many
additional encoding strings. 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). Compare 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
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 representable 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 soelim(1); 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.
-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 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.
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 tarball groff-1.23.0.tar.gz fetched
from ⟨https://ftp.gnu.org/gnu/groff/⟩ on 2026-01-16. 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 1.23.0 2 July 2023 preconv(1)
Pages that refer to this page: groff(1)
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HTML rendering created 2026-01-16 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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