1FOLD(1P) POSIX Programmer's Manual FOLD(1P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
12 fold — filter for folding lines
13
15 fold [-bs] [-w width] [file...]
16
18 The fold utility is a filter that shall fold lines from its input
19 files, breaking the lines to have a maximum of width column positions
20 (or bytes, if the -b option is specified). Lines shall be broken by the
21 insertion of a <newline> such that each output line (referred to later
22 in this section as a segment) is the maximum width possible that does
23 not exceed the specified number of column positions (or bytes). A line
24 shall not be broken in the middle of a character. The behavior is unde‐
25 fined if width is less than the number of columns any single character
26 in the input would occupy.
27 If the <carriage-return>, <backspace>, or <tab> characters are encoun‐
29 tered in the input, and the -b option is not specified, they shall be
30 treated specially:
31 <backspace>
33 The current count of line width shall be decremented by one,
34 although the count never shall become negative. The fold
35 utility shall not insert a <newline> immediately before or
36 after any <backspace>, unless the following character has a
37 width greater than 1 and would cause the line width to exceed
38 width.
39 <carriage-return>
41 The current count of line width shall be set to zero. The
42 fold utility shall not insert a <newline> immediately before
43 or after any <carriage-return>.
44 <tab> Each <tab> encountered shall advance the column position
46 pointer to the next tab stop. Tab stops shall be at each col‐
47 umn position n such that n modulo 8 equals 1.
48
50 The fold utility shall conform to the Base Definitions volume of
51 POSIX.1‐2017, Section 12.2, Utility Syntax Guidelines.
52 The following options shall be supported:
54 -b Count width in bytes rather than column positions.
56 -s If a segment of a line contains a <blank> within the first
58 width column positions (or bytes), break the line after the
59 last such <blank> meeting the width constraints. If there is
60 no <blank> meeting the requirements, the -s option shall have
61 no effect for that output segment of the input line.
62 -w width Specify the maximum line length, in column positions (or
64 bytes if -b is specified). The results are unspecified if
65 width is not a positive decimal number. The default value
66 shall be 80.
67
69 The following operand shall be supported:
70 file A pathname of a text file to be folded. If no file operands
72 are specified, the standard input shall be used.
73
75 The standard input shall be used if no file operands are specified, and
76 shall be used if a file operand is '-' and the implementation treats
77 the '-' as meaning standard input. Otherwise, the standard input shall
78 not be used. See the INPUT FILES section.
79
81 If the -b option is specified, the input files shall be text files
82 except that the lines are not limited to {LINE_MAX} bytes in length. If
83 the -b option is not specified, the input files shall be text files.
84
86 The following environment variables shall affect the execution of fold:
87 LANG Provide a default value for the internationalization vari‐
89 ables that are unset or null. (See the Base Definitions vol‐
90 ume of POSIX.1‐2017, Section 8.2, Internationalization Vari‐
91 ables for the precedence of internationalization variables
92 used to determine the values of locale categories.)
93 LC_ALL If set to a non-empty string value, override the values of
95 all the other internationalization variables.
96 LC_CTYPE Determine the locale for the interpretation of sequences of
98 bytes of text data as characters (for example, single-byte as
99 opposed to multi-byte characters in arguments and input
100 files), and for the determination of the width in column
101 positions each character would occupy on a constant-width
102 font output device.
103 LC_MESSAGES
105 Determine the locale that should be used to affect the format
106 and contents of diagnostic messages written to standard
107 error.
108 NLSPATH Determine the location of message catalogs for the processing
110 of LC_MESSAGES.
111
113 Default.
114
116 The standard output shall be a file containing a sequence of characters
117 whose order shall be preserved from the input files, possibly with
118 inserted <newline> characters.
119
121 The standard error shall be used only for diagnostic messages.
122
124 None.
125
127 None.
128
130 The following exit values shall be returned:
131 0 All input files were processed successfully.
133 >0 An error occurred.
135
137 Default.
138 The following sections are informative.
140
142 The cut and fold utilities can be used to create text files out of
143 files with arbitrary line lengths. The cut utility should be used when
144 the number of lines (or records) needs to remain constant. The fold
145 utility should be used when the contents of long lines need to be kept
146 contiguous.
147 The fold utility is frequently used to send text files to printers that
149 truncate, rather than fold, lines wider than the printer is able to
150 print (usually 80 or 132 column positions).
151
153 An example invocation that submits a file of possibly long lines to the
154 printer (under the assumption that the user knows the line width of the
155 printer to be assigned by lp):
156 fold -w 132 bigfile | lp
159
161 Although terminal input in canonical processing mode requires the erase
162 character (frequently set to <backspace>) to erase the previous charac‐
163 ter (not byte or column position), terminal output is not buffered and
164 is extremely difficult, if not impossible, to parse correctly; the
165 interpretation depends entirely on the physical device that actually
166 displays/prints/stores the output. In all known internationalized
167 implementations, the utilities producing output for mixed column-width
168 output assume that a <backspace> character backs up one column position
169 and outputs enough <backspace> characters to return to the start of the
170 character when <backspace> is used to provide local line motions to
171 support underlining and emboldening operations. Since fold without the
172 -b option is dealing with these same constraints, <backspace> is always
173 treated as backing up one column position rather than backing up one
174 character.
175 Historical versions of the fold utility assumed 1 byte was one charac‐
177 ter and occupied one column position when written out. This is no
178 longer always true. Since the most common usage of fold is believed to
179 be folding long lines for output to limited-length output devices, this
180 capability was preserved as the default case. The -b option was added
181 so that applications could fold files with arbitrary length lines into
182 text files that could then be processed by the standard utilities. Note
183 that although the width for the -b option is in bytes, a line is never
184 split in the middle of a character. (It is unspecified what happens if
185 a width is specified that is too small to hold a single character found
186 in the input followed by a <newline>.)
187 The tab stops are hardcoded to be every eighth column to meet histori‐
189 cal practice. No new method of specifying other tab stops was invented.
190
192 None.
193
195 cut
196 The Base Definitions volume of POSIX.1‐2017, Chapter 8, Environment
198 Variables, Section 12.2, Utility Syntax Guidelines
199
201 Portions of this text are reprinted and reproduced in electronic form
202 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
203 table Operating System Interface (POSIX), The Open Group Base Specifi‐
204 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
205 Electrical and Electronics Engineers, Inc and The Open Group. In the
206 event of any discrepancy between this version and the original IEEE and
207 The Open Group Standard, the original IEEE and The Open Group Standard
208 is the referee document. The original Standard can be obtained online
209 at http://www.opengroup.org/unix/online.html .
210 Any typographical or formatting errors that appear in this page are
212 most likely to have been introduced during the conversion of the source
213 files to man page format. To report such errors, see https://www.ker‐
214 nel.org/doc/man-pages/reporting_bugs.html .
215IEEE/The Open Group 2017 FOLD(1P)