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.TH FOLD "1P" 2013 "IEEE/The Open Group" "POSIX Programmer's Manual"

.SH PROLOG

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.

.SH NAME

fold

\(em filter for folding lines

.SH SYNOPSIS

.LP

.nf

fold \fB[\fR\(mibs\fB] [\fR\(miw \fIwidth\fB] [\fIfile\fR...\fB]\fR

.fi

.SH DESCRIPTION

The

.IR fold

utility is a filter that shall fold lines from its input files,

breaking the lines to have a maximum of

.IR width

column positions (or bytes, if the

.BR \(mib

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 \fIsegment\fP) 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

.IR width

is less than the number of columns any single character in the input

would occupy.

.P

If the

<carriage-return>,

<backspace>,

or

<tab>

characters are encountered in the input, and the

.BR \(mib

option is not specified, they shall be treated specially:

.IP <backspace> 10

The current count of line width shall be decremented by one, although

the count never shall become negative. The

.IR 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

.IR width .

.IP <carriage-return> 10

.br

The current count of line width shall be set to zero. The

.IR fold

utility shall not insert a

<newline>

immediately before or after any

<carriage-return>.

.IP <tab> 10

Each

<tab>

encountered shall advance the column position pointer to the next tab

stop. Tab stops shall be at each column position

.IR n

such that

.IR n

modulo 8 equals 1.

.SH OPTIONS

The

.IR fold

utility shall conform to the Base Definitions volume of POSIX.1\(hy2008,

.IR "Section 12.2" ", " "Utility Syntax Guidelines".

.P

The following options shall be supported:

.IP "\fB\(mib\fR" 10

Count

.IR width

in bytes rather than column positions.

.IP "\fB\(mis\fR" 10

If a segment of a line contains a

<blank>

within the first

.IR 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

.BR \(mis

option shall have no effect for that output segment of the input line.

.IP "\fB\(miw\ \fIwidth\fR" 10

Specify the maximum line length, in column positions (or bytes if

.BR \(mib

is specified). The results are unspecified if

.IR width

is not a positive decimal number. The default value shall be 80.

.SH OPERANDS

The following operand shall be supported:

.IP "\fIfile\fR" 10

A pathname of a text file to be folded. If no

.IR file

operands are specified, the standard input shall be used.

.SH STDIN

The standard input shall be used if no

.IR file

operands are specified, and shall be used if a

.IR file

operand is

.BR '\(mi' 

and the implementation treats the

.BR '\(mi' 

as meaning standard input.

Otherwise, the standard input shall not be used.

See the INPUT FILES section.

.SH "INPUT FILES"

If the

.BR \(mib

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

.BR \(mib

option is not specified, the input files shall be text files.

.SH "ENVIRONMENT VARIABLES"

The following environment variables shall affect the execution of

.IR fold :

.IP "\fILANG\fP" 10

Provide a default value for the internationalization variables that are

unset or null. (See the Base Definitions volume of POSIX.1\(hy2008,

.IR "Section 8.2" ", " "Internationalization Variables"

for the precedence of internationalization variables used to determine

the values of locale categories.)

.IP "\fILC_ALL\fP" 10

If set to a non-empty string value, override the values of all the

other internationalization variables.

.IP "\fILC_CTYPE\fP" 10

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.

.IP "\fILC_MESSAGES\fP" 10

.br

Determine the locale that should be used to affect the format and

contents of diagnostic messages written to standard error.

.IP "\fINLSPATH\fP" 10

Determine the location of message catalogs for the processing of

.IR LC_MESSAGES .

.SH "ASYNCHRONOUS EVENTS"

Default.

.SH 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>

characters.

.SH STDERR

The standard error shall be used only for diagnostic messages.

.SH "OUTPUT FILES"

None.

.SH "EXTENDED DESCRIPTION"

None.

.SH "EXIT STATUS"

The following exit values shall be returned:

.IP "\00" 6

All input files were processed successfully.

.IP >0 6

An error occurred.

.SH "CONSEQUENCES OF ERRORS"

Default.

.LP

.IR "The following sections are informative."

.SH "APPLICATION USAGE"

The

.IR cut

and

.IR fold

utilities can be used to create text files out of files with arbitrary

line lengths. The

.IR cut

utility should be used when the number of lines (or records) needs to

remain constant. The

.IR fold

utility should be used when the contents of long lines need to be kept

contiguous.

.P

The

.IR 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).

.SH 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

.IR lp ):

.sp

.RS 4

.nf

\fB

fold \(miw 132 bigfile | lp

.fi \fR

.P

.RE

.SH 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 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

.IR fold

without the

.BR \(mib

option is dealing with these same constraints,

<backspace>

is always treated as backing up one column position rather than backing

up one character.

.P

Historical versions of the

.IR 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

.IR fold

is believed to be folding long lines for output to limited-length

output devices, this capability was preserved as the default case. The

.BR \(mib

option was added so that applications could

.IR 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

.BR \(mib

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>.)

.P

The tab stops are hardcoded to be every eighth column to meet

historical practice. No new method of specifying other tab stops was

invented.

.SH "FUTURE DIRECTIONS"

None.

.SH "SEE ALSO"

.IR "\fIcut\fR\^"

.P

The Base Definitions volume of POSIX.1\(hy2008,

.IR "Chapter 8" ", " "Environment Variables",

.IR "Section 12.2" ", " "Utility Syntax Guidelines"

.SH COPYRIGHT

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 .