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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 (con-
sult 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 behav-
ior 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.

OPTIONS
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.

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 only if no file operands are specified. 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 IEEE Std 1003.1-2001, Section 8.2, Internationalization Variables for the precedence of international-
ization 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 stan-
dard 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>s.

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 dis-
plays/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 charac-
ter. (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.