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PROGRAM:

NAME

tabs — set terminal tabs

SYNOPSIS

tabs [−n|−a|−a2|−c|−c2|−c3|−f|−p|−s|−u] [−T type]

tabs [−T type] n[[sep[+]n]...]

DESCRIPTION

The tabs utility shall display a series of characters that first clears the hardware
terminal tab settings and then initializes the tab stops at the specified positions and
optionally adjusts the margin.

The phrase ``tab-stop position N'' shall be 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. The maximum number of tab stops allowed is terminal-
dependent.

It need not be possible to implement tabs on certain terminals. If the terminal type
obtained from the TERM environment variable or −T option represents such a terminal, an
appropriate diagnostic message shall be written to standard error and tabs shall exit with
a status greater than zero.

OPTIONS

The tabs utility shall conform to the Base Definitions volume of POSIX.1‐2008, Section
12.2, Utility Syntax Guidelines, except for various extensions: the options −a2, −c2, and
−c3 are multi-character.

The following options shall be supported:

−n Specify repetitive tab stops separated by a uniform number of column positions,
n, where n is a single-digit decimal number. The default usage of tabs with no
arguments shall be equivalent to tabs −8. When −0 is used, the tab stops shall
be cleared and no new ones set.

−a 1,10,16,36,72
Assembler, applicable to some mainframes.

−a2 1,10,16,40,72
Assembler, applicable to some mainframes.

−c 1,8,12,16,20,55
COBOL, normal format.

−c2 1,6,10,14,49
COBOL, compact format (columns 1 to 6 omitted).

−c3 1,6,10,14,18,22,26,30,34,38,42,46,50,54,58,62,67
COBOL compact format (columns 1 to 6 omitted), with more tabs than −c2.

−f 1,7,11,15,19,23
FORTRAN

−p 1,5,9,13,17,21,25,29,33,37,41,45,49,53,57,61
PL/1

−s 1,10,55
SNOBOL

−u 1,12,20,44
Assembler, applicable to some mainframes.

−T type Indicate the type of terminal. If this option is not supplied and the TERM
variable is unset or null, an unspecified default terminal type shall be used.
The setting of type shall take precedence over the value in TERM.

OPERANDS

The following operand shall be supported:

n[[sep[+]n]...]
A single command line argument that consists of one or more tab-stop values (n)
separated by a separator character (sep) which is either a <comma> or a <blank>
character. The application shall ensure that the tab-stop values are positive
decimal integers in strictly ascending order. If any tab-stop value (except the
first one) is preceded by a <plus-sign>, it is taken as an increment to be added
to the previous value. For example, the tab lists 1,10,20,30 and "110+10+10" are
considered to be identical.

STDIN

Not used.

INPUT FILES

None.

ENVIRONMENT VARIABLES

The following environment variables shall affect the execution of tabs:

LANG 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.)

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).

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.

TERM Determine the terminal type. If this variable is unset or null, and if the −T
option is not specified, an unspecified default terminal type shall be used.

ASYNCHRONOUS EVENTS

Default.

STDOUT

If standard output is a terminal, the appropriate sequence to clear and set the tab stops
may be written to standard output in an unspecified format. If standard output is not a
terminal, undefined results occur.

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

This utility makes use of the terminal's hardware tabs and the stty tabs option.

This utility is not recommended for application use.

Some integrated display units might not have escape sequences to set tab stops, but may be
set by internal system calls. On these terminals, tabs works if standard output is
directed to the terminal; if output is directed to another file, however, tabs fails.

EXAMPLES

None.

RATIONALE

Consideration was given to having the tput utility handle all of the functions described
in tabs. However, the separate tabs utility was retained because it seems more intuitive
to use a command named tabs than tput with a new option. The tput utility does not support
setting or clearing tabs, and no known historical version of tabs supports the capability
of setting arbitrary tab stops.

The System V tabs interface is very complex; the version in this volume of POSIX.1‐2008
has a reduced feature list, but many of the features omitted were restored as part of the
XSI option even though the supported languages and coding styles are primarily historical.

There was considerable sentiment for specifying only a means of resetting the tabs back to
a known state—presumably the ``standard'' of tabs every eight positions. The following
features were omitted:

* Setting tab stops via the first line in a file, using −−file. Since even the SVID has
no complete explanation of this feature, it is doubtful that it is in widespread use.

In an early proposal, a −t tablist option was added for consistency with expand; this was
later removed when inconsistencies with the historical list of tabs were identified.

Consideration was given to adding a −p option that would output the current tab settings
so that they could be saved and then later restored. This was not accepted because
querying the tab stops of the terminal is not a capability in historical terminfo or
termcap facilities and might not be supported on a wide range of terminals.

FUTURE DIRECTIONS

None.

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