Ubuntu Manpage: fold - filter for folding lines

NAME

       fold - filter for folding lines

SYNOPSIS

       fold [-bs][-w width][file...]

DESCRIPTION

       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>s, <backspace>s, or <tab>s 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>.

       <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.

OPTIONS

       The fold utility shall conform to the Base Definitions  volume  of  IEEE Std 1003.1-2001,  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.

OPERANDS

       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.

STDIN

       The standard input shall be used only if no file operands are specified. See the INPUT FILES section.

INPUT FILES

       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.

ENVIRONMENT VARIABLES

       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 IEEE Std 1003.1-2001, 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 .

ASYNCHRONOUS EVENTS

       Default.

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>s.

STDERR

       The standard error shall be used only for diagnostic messages.

OUTPUT FILES

       None.

EXTENDED DESCRIPTION

       None.

EXIT STATUS

       The following exit values shall be returned:

        0     All input files were processed successfully.

       >0     An error occurred.

CONSEQUENCES OF ERRORS

       Default.

       The following sections are informative.

APPLICATION USAGE

       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).

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 lp):

              fold -w 132 bigfile | lp

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> backs up one column position and outputs enough <backspace>s 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.

FUTURE DIRECTIONS

       None.

COPYRIGHT

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition,
       Standard  for  Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 6, Copyright (C) 2001-2003 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 .

IEEE/The Open Group                                   2003                                               FOLD(P)