The Open Group Base Specifications Issue 7
IEEE Std 1003.1-2008, 2016 Edition
Copyright © 2001-2016 The IEEE and The Open Group


fold - filter for folding lines


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>, <backspace>, or <tab> characters are encountered in the input, and the -b option is not specified, they shall be treated specially:

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.
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>.
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 XBD Utility Syntax Guidelines .

The following options shall be supported:

Count width in bytes rather than column positions.
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:

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 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.


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:

Provide a default value for the internationalization variables that are unset or null. (See XBD Internationalization Variables for the precedence of internationalization variables used to determine the values of locale categories.)
If set to a non-empty string value, override the values of all the other internationalization variables.
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.
Determine the locale that should be used to affect the format and contents of diagnostic messages written to standard error.
[XSI] [Option Start] Determine the location of message catalogs for the processing of LC_MESSAGES. [Option End]




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.


The standard error shall be used only for diagnostic messages.






The following exit values shall be returned:

All input files were processed successfully.
An error occurred.



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):

fold -w 132 bigfile | 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> 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.





XBD Environment Variables, Utility Syntax Guidelines


First released in Issue 4.

Issue 6

The normative text is reworded to avoid use of the term "must" for application requirements.

Issue 7

Austin Group Interpretation 1003.1-2001 #092 is applied.

Austin Group Interpretation 1003.1-2001 #204 is applied, updating the DESCRIPTION to clarify when a <newline> can be inserted before or after a <backspace>.

SD5-XCU-ERN-97 is applied, updating the SYNOPSIS.

End of informative text.


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