фильтр для линий сгиба (filter for folding lines)
Пролог (Prolog)
This manual page is part of the POSIX Programmer's Manual. The
Linux implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
Имя (Name)
fold — filter for folding lines
Синопсис (Synopsis)
fold [-bs] [-w width] [file...]
Описание (Description)
The fold utility is a filter that shall fold lines from its input
files, breaking the lines to have a maximum of width column
positions (or bytes, if the -b option is specified). Lines shall
be broken by the insertion of a <newline> such that each output
line (referred to later in this section as a segment) is the
maximum width possible that does not exceed the specified number
of column positions (or bytes). A line shall not be broken in the
middle of a character. The behavior is undefined if width is less
than the number of columns any single character in the input
would occupy.
If the <carriage-return>, <backspace>, or <tab> characters are
encountered in the input, and the -b option is not specified,
they shall be treated specially:
<backspace>
The current count of line width shall be decremented by
one, although the count never shall become negative.
The fold utility shall not insert a <newline>
immediately before or after any <backspace>, unless the
following character has a width greater than 1 and
would cause the line width to exceed width.
<carriage-return>
The current count of line width shall be set to zero.
The fold utility shall not insert a <newline>
immediately before or after any <carriage-return>.
<tab> Each <tab> encountered shall advance the column
position pointer to the next tab stop. Tab stops shall
be at each column position n such that n modulo 8
equals 1.
Параметры (Options)
The fold utility shall conform to the Base Definitions volume of
POSIX.1‐2017, Section 12.2, Utility Syntax Guidelines.
The following options shall be supported:
-b Count width in bytes rather than column positions.
-s If a segment of a line contains a <blank> within the
first width column positions (or bytes), break the line
after the last such <blank> meeting the width
constraints. If there is no <blank> meeting the
requirements, the -s option shall have no effect for
that output segment of the input line.
-w width Specify the maximum line length, in column positions
(or bytes if -b is specified). The results are
unspecified if width is not a positive decimal number.
The default value shall be 80.
Операнды (Operands)
The following operand shall be supported:
file A pathname of a text file to be folded. If no file
operands are specified, the standard input shall be
used.
Стандартный ввод (Stdin)
The standard input shall be used if no file operands are
specified, and shall be used if a file operand is '-' and the
implementation treats the '-' as meaning standard input.
Otherwise, the standard input shall not be used. See the INPUT
FILES section.
Входные файлы (Input files)
If the -b option is specified, the input files shall be text
files except that the lines are not limited to {LINE_MAX} bytes
in length. If the -b option is not specified, the input files
shall be text files.
Переменные окружения (Environment variables)
The following environment variables shall affect the execution of
fold:
LANG Provide a default value for the internationalization
variables that are unset or null. (See the Base
Definitions volume of POSIX.1‐2017, Section 8.2,
Internationalization Variables for the precedence of
internationalization variables used to determine the
values of locale categories.)
LC_ALL If set to a non-empty string value, override the values
of all the other internationalization variables.
LC_CTYPE Determine the locale for the interpretation of
sequences of bytes of text data as characters (for
example, single-byte as opposed to multi-byte
characters in arguments and input files), and for the
determination of the width in column positions each
character would occupy on a constant-width font output
device.
LC_MESSAGES
Determine the locale that should be used to affect the
format and contents of diagnostic messages written to
standard error.
NLSPATH Determine the location of message catalogs for the
processing of LC_MESSAGES.
Асинхронные события (Asynchronous events)
Default.
Стандартный вывод (Stdout)
The standard output shall be a file containing a sequence of
characters whose order shall be preserved from the input files,
possibly with inserted <newline> characters.
Стандартный вывод сообщений (Stderr)
The standard error shall be used only for diagnostic messages.
Выходные файлы (Output files)
None.
Расширенное описание (Extended description)
None.
Статус выхода (Exit)
The following exit values shall be returned:
0 All input files were processed successfully.
>0 An error occurred.
Последствия ошибок (Consequences of errors)
Default.
The following sections are informative.
Использование в приложениях (Application usage)
The cut and fold utilities can be used to create text files out
of files with arbitrary line lengths. The cut utility should be
used when the number of lines (or records) needs to remain
constant. The fold utility should be used when the contents of
long lines need to be kept contiguous.
The fold utility is frequently used to send text files to
printers that truncate, rather than fold, lines wider than the
printer is able to print (usually 80 or 132 column positions).
Примеры (Examples)
An example invocation that submits a file of possibly long lines
to the printer (under the assumption that the user knows the line
width of the printer to be assigned by lp):
fold -w 132 bigfile | lp
Обоснование (Rationale)
Although terminal input in canonical processing mode requires the
erase character (frequently set to <backspace>) to erase the
previous character (not byte or column position), terminal output
is not buffered and is extremely difficult, if not impossible, to
parse correctly; the interpretation depends entirely on the
physical device that actually displays/prints/stores the output.
In all known internationalized implementations, the utilities
producing output for mixed column-width output assume that a
<backspace> character backs up one column position and outputs
enough <backspace> characters to return to the start of the
character when <backspace> is used to provide local line motions
to support underlining and emboldening operations. Since fold
without the -b option is dealing with these same constraints,
<backspace> is always treated as backing up one column position
rather than backing up one character.
Historical versions of the fold utility assumed 1 byte was one
character and occupied one column position when written out. This
is no longer always true. Since the most common usage of fold is
believed to be folding long lines for output to limited-length
output devices, this capability was preserved as the default
case. The -b option was added so that applications could fold
files with arbitrary length lines into text files that could then
be processed by the standard utilities. Note that although the
width for the -b option is in bytes, a line is never split in the
middle of a character. (It is unspecified what happens if a
width is specified that is too small to hold a single character
found in the input followed by a <newline>.)
The tab stops are hardcoded to be every eighth column to meet
historical practice. No new method of specifying other tab stops
was invented.
Будущие направления (Future directions)
None.
Смотри также (See also)
cut(1p)
The Base Definitions volume of POSIX.1‐2017, Chapter 8,
Environment Variables, Section 12.2, Utility Syntax Guidelines
