unexpand man page


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.

unexpand — convert spaces to tabs


unexpand [-a|-t tablist] [file...]


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.


The unexpand utility shall conform to the Base Definitions volume of POSIX.1-2008, Section 12.2, Utility Syntax Guidelines.

The following options shall be supported:


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.


The following operand shall be supported:


A pathname of a text file to be used as input.


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:


Provide a default value for the internationalization variables that are unset or null. (See the Base Definitions volume of POSIX.1-2008, 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> and <space> characters, 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.

Asynchronous Events



The standard output shall be equivalent to the input files with the specified <space>-to-<tab> conversions.


The standard error shall be used only for diagnostic messages.

Output Files


Extended Description


Exit Status

The following exit values shall be returned:


Successful completion.


An error occurred.

Consequences of Errors


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.




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. 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


See Also

expand, tabs

The Base Definitions volume of POSIX.1-2008, Chapter 8, Environment Variables, Section 12.2, Utility Syntax Guidelines

Referenced By

expand(1p), tabs(1p).

2013 IEEE/The Open Group POSIX Programmer's Manual