Section: POSIX Programmer's Manual (1P)
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.
--- filter for folding lines
fold [-bs] [-w width] [file...]
utility is a filter that shall fold lines from its input files,
breaking the lines to have a maximum of
column positions (or bytes, if the
option is specified). Lines shall be broken by the insertion of a
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
is less than the number of columns any single character in the input
characters are encountered in the input, and the
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
utility shall not insert a
immediately before or after any
unless the following character has a width greater than 1 and would
cause the line width to exceed
The current count of line width shall be set to zero. The
utility shall not insert a
immediately before or after any
encountered shall advance the column position pointer to the next tab
stop. Tab stops shall be at each column position
modulo 8 equals 1.
utility shall conform to the Base Definitions volume of POSIX.1-2008,
, Utility Syntax Guidelines.
The following options shall be supported:
in bytes rather than column positions.
If a segment of a line contains a
within the first
column positions (or bytes), break the line after the last such
meeting the width constraints. If there is no
meeting the requirements, the
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
is specified). The results are unspecified if
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
operands are specified, the standard input shall be used.
The standard input shall be used if no
operands are specified, and shall be used if a
and the implementation treats the
as meaning standard input.
Otherwise, the standard input shall not be used.
See the INPUT FILES section.
option is specified, the input files shall be text files except that the
lines are not limited to
bytes in length. If the
option is not specified, the input files shall be text files.
The following environment variables shall affect the execution of
Provide a default value for the internationalization variables that are
unset or null. (See the Base Definitions volume of POSIX.1-2008,
Section 8.2, 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.
Determine the location of message catalogs for the processing of
The standard output shall be a file containing a sequence of characters
whose order shall be preserved from the input files, possibly with
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.
CONSEQUENCES OF ERRORS
The following sections are informative.
utilities can be used to create text files out of files with arbitrary
line lengths. The
utility should be used when the number of lines (or records) needs to
remain constant. The
utility should be used when the contents of long lines need to be kept
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
fold -w 132 bigfile | lp
Although terminal input in canonical processing mode requires the erase
character (frequently set to
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
character backs up one column position and outputs enough
characters to return to the start of the character when
is used to provide local line motions to support underlining and
emboldening operations. Since
option is dealing with these same constraints,
is always treated as backing up one column position rather than backing
up one character.
Historical versions of the
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
is believed to be folding long lines for output to limited-length
output devices, this capability was preserved as the default case. The
option was added so that applications could
files with arbitrary length lines into text files that could then be
processed by the standard utilities. Note that although the width for
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
The tab stops are hardcoded to be every eighth column to meet
historical practice. No new method of specifying other tab stops was
The Base Definitions volume of POSIX.1-2008,
Chapter 8, Environment Variables,
Section 12.2, Utility Syntax Guidelines
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
Electrical 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
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