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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 |
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.1‐2008, 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.1‐2008, 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.1‐2008, 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 http://www.unix.org/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 2013 FOLD(1P)
Pages that refer to this page: cut(1p), id(1p)