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PROLOG | NAME | SYNOPSIS | DESCRIPTION | OPTIONS | OPERANDS | STDIN | INPUT FILES | ENVIRONMENT VARIABLES | ASYNCHRONOUS EVENTS | STDOUT | STDERR | OUTPUT FILES | EXTENDED DESCRIPTION | EXIT STATUS | CONSEQUENCES OF ERRORS | APPLICATION USAGE | EXAMPLES | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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FOLD(1P) POSIX Programmer's Manual FOLD(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.
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:
<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.
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.
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.
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:
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.
Default.
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.
None.
None.
The following exit values shall be returned:
0 All input files were processed successfully.
>0 An error occurred.
Default.
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.
None.
cut(1p)
The Base Definitions volume of POSIX.1‐2017, Chapter 8,
Environment Variables, Section 12.2, Utility Syntax Guidelines
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, 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 .
IEEE/The Open Group 2017 FOLD(1P)
Pages that refer to this page: cut(1p), id(1p)
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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