Originální popis anglicky:
fold - filter for folding lines
Návod, kniha: POSIX Programmer's Manual
fold [-bs][-w
width][file ...]
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>s, <backspace>s, or <tab>s 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>.
- <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.
The
fold utility shall conform to the Base Definitions volume of
IEEE Std 1003.1-2001, 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.
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.
The standard input shall be used only if no
file operands are specified.
See the INPUT FILES section.
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.
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
IEEE Std 1003.1-2001, 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 .
Default.
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>s.
The standard error shall be used only for diagnostic messages.
None.
None.
The following exit values shall be returned:
- 0
- All input files were processed successfully.
- >0
- An error occurred.
Default.
The following sections are informative.
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).
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):
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> backs up one column position and outputs
enough <backspace>s 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.
None.
cut
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard, the original
IEEE and The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.opengroup.org/unix/online.html
.
