FOLD(1P) POSIX Programmer's Manual FOLD(1P)
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.
NAME
fold -- filter for folding lines
SYNOPSIS
fold [-bs] [-w width] [file...]
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 unde-
fined 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 encoun-
tered 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 col-
umn position n such that n modulo 8 equals 1.
OPTIONS
The fold utility shall conform to the Base Definitions volume of
POSIX.1-2008, 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.
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.
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.
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.
ENVIRONMENT VARIABLES
The following environment variables shall affect the execution of fold:
LANG Provide a default value for the internationalization vari-
ables that are unset or null. (See the Base Definitions vol-
ume of POSIX.1-2008, Section 8.2, Internationalization Vari-
ables 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.
ASYNCHRONOUS EVENTS
Default.
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.
STDERR
The standard error shall be used only for diagnostic messages.
OUTPUT FILES
None.
EXTENDED DESCRIPTION
None.
EXIT STATUS
The following exit values shall be returned:
0 All input files were processed successfully.
>0 An error occurred.
CONSEQUENCES OF ERRORS
Default.
The following sections are informative.
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).
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
RATIONALE
Although terminal input in canonical processing mode requires the erase
character (frequently set to <backspace>) to erase the previous charac-
ter (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 charac-
ter 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 histori-
cal practice. No new method of specifying other tab stops was invented.
FUTURE DIRECTIONS
None.
SEE ALSO
cut
The Base Definitions volume of POSIX.1-2008, Chapter 8, Environment
Variables, Section 12.2, Utility Syntax Guidelines
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri-
cal and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the source
files to man page format. To report such errors, see https://www.ker-
nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 FOLD(1P)
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