FOLD(1POSIX)

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 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.

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.

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 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.

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 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.

FUTURE DIRECTIONS
None.

SEE ALSO
cut

The Base Definitions volume of POSIX.1-2017, 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 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .

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.kernel.org/doc/man-pages/reporting_bugs.html .