The unexpand utility shall copy files or standard input to standard
output, converting <blank>s at the beginning of each line into the
maximum number of <tab>s followed by the minimum number of
<space>s needed to fill the same column positions originally filled by
the translated <blank>s. 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.
The unexpand 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:
In addition to translating <blank>s at the beginning
of each line, translate all sequences of two or more <blank>s
immediately preceding a tab stop to the maximum number of <tab>s
followed by the minimum number of <space>s needed to fill the same
column positions originally filled by the translated <blank>s.
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>s or commas, 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
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
The following environment variables shall affect the execution of
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.)
If set to a non-empty string value, override the values of
all the other internationalization variables.
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>s and <space>s, and for the determination of the width in
column positions each character would occupy on an output device.
Determine the locale that should be used to affect the
format and contents of diagnostic messages written to standard error.
Determine the location of message catalogs for the
processing of LC_MESSAGES.
One non-intuitive aspect of unexpand is its restriction to leading spaces
when neither -a nor -t is specified. Users who always want to
convert all spaces in a file can easily alias unexpand to use the
-a or -t 8 option.
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>s 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. 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.
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