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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
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NAME
unexpand — convert spaces to tabs
SYNOPSIS
unexpand [-a|-t tablist] [file...]
DESCRIPTION
The unexpand utility shall copy files or standard input to standard output, converting
<blank> characters at the beginning of each line into the maximum number of <tab>
characters followed by the minimum number of <space> characters needed to fill the same
column positions originally filled by the translated <blank> characters. By default,
tabstops shall be set at every eighth column position. Each <backspace> shall be copied to
the output, and shall cause the column position count for tab calculations to be
decremented; the count shall never be decremented to a value less than one.
OPTIONS
The unexpand utility shall conform to the Base Definitions volume of POSIX.1‐2017, Section
12.2, Utility Syntax Guidelines.
The following options shall be supported:
-a In addition to translating <blank> characters at the beginning of each line,
translate all sequences of two or more <blank> characters immediately preceding
a tab stop to the maximum number of <tab> characters followed by the minimum
number of <space> characters needed to fill the same column positions originally
filled by the translated <blank> characters.
-t tablist
Specify the tab stops. The application shall ensure that the tablist option-
argument is a single argument consisting of a single positive decimal integer or
multiple positive decimal integers, separated by <blank> or <comma> characters,
in ascending order. If a single number is given, tabs shall be set tablist
column positions apart instead of the default 8. If multiple numbers are given,
the tabs shall be set at those specific column positions.
The application shall ensure that each tab-stop position N is an integer value
greater than zero, and the list shall be in strictly ascending order. This is
taken to mean that, from the start of a line of output, tabbing to position N
shall cause the next character output to be in the (N+1)th column position on
that line. When the -t option is not specified, the default shall be the
equivalent of specifying -t 8 (except for the interaction with -a, described
below).
No <space>-to-<tab> conversions shall occur for characters at positions beyond
the last of those specified in a multiple tab-stop list.
When -t is specified, the presence or absence of the -a option shall be ignored;
conversion shall not be limited to the processing of leading <blank> characters.
OPERANDS
The following operand shall be supported:
file A pathname of a text file to be used as input.
STDIN
See the INPUT FILES section.
INPUT FILES
The input files shall be text files.
ENVIRONMENT VARIABLES
The following environment variables shall affect the execution of unexpand:
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), the processing of <tab> and <space> characters, and
for the determination of the width in column positions each character would
occupy on an 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 equivalent to the input files with the specified
<space>-to-<tab> conversions.
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 Successful completion.
>0 An error occurred.
CONSEQUENCES OF ERRORS
Default.
The following sections are informative.
APPLICATION USAGE
One non-intuitive aspect of unexpand is its restriction to leading <space> characters when
neither -a nor -t is specified. Users who always want to convert all <space> characters in
a file can easily alias unexpand to use the -a or -t 8 option.
EXAMPLES
None.
RATIONALE
On several occasions, consideration was given to adding a -t option to the unexpand
utility to complement the -t in expand (see expand). The historical intent of unexpand
was to translate multiple <blank> characters into tab stops, where tab stops were a
multiple of eight column positions on most UNIX systems. An early proposal omitted -t
because it seemed outside the scope of the User Portability Utilities option; it was not
described in any of the base documents for IEEE Std 1003.2‐1992. However, hard-coding tab
stops every eight columns was not suitable for the international community and broke
historical precedents for some vendors in the FORTRAN community, so -t was restored in
conjunction with the list of valid extension categories considered by the standard
developers. Thus, unexpand is now the logical converse of expand.
FUTURE DIRECTIONS
None.
SEE ALSO
expand, tabs
The Base Definitions volume of POSIX.1‐2017, Chapter 8, Environment Variables, Section
12.2, Utility Syntax Guidelines
COPYRIGHT
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
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