The tabs program clears and sets tab-stops on the terminal.
This uses the terminfo clear_all_tabs and set_tab capabilities.
If either is absent, tabs is unable to clear/set tab-stops.
The terminal should be configured to use hard tabs, e.g.,
.sp
.in +4
stty tab0
Like clear(1), tabs writes to the standard output. You can redirect the standard output to a file (which prevents tabs from actually changing the tabstops), and later cat the file to the screen, setting tabstops at that point.
These are hardware tabs, which cannot be queried rapidly by applications
running in the terminal, if at all.
Curses and other full-screen applications may use hardware tabs
in optimizing their output to the terminal.
If the hardware tabstops differ from the information in the terminal
database, the result is unpredictable.
Before running curses programs,
you should either reset tab-stops to the standard interval
.sp
.in +4
tabs -8
or use the reset program, since the normal initialization sequences do not ensure that tab-stops are reset.
The tabs program processes a single list of tab stops. The last option to be processed which defines a list is the one that determines the list to be processed.
Use ``-0'' to clear all tabs.
Use ``-8'' to set tabs to the standard interval.
tabs 1,6,11,16,21
tabs 1 6 11 16 21
Use a ``+'' to treat a number
as an increment relative to the previous value,
e.g.,
.sp
.in +4
tabs 1,+5,+5,+5,+5
which is equivalent to the 1,6,11,16,21 example.
IEEE Std 1003.1/The Open Group Base Specifications Issue 7 (POSIX.1-2008)
describes a tabs utility.
However
.IP • 4
This standard describes a +m option, to set a terminal's left-margin.
Very few of the entries in the terminal database provide the
smgl (set_left_margin) or
smglp (set_left_margin_parm)
capability needed to support the feature.
.IP • 4
There is no counterpart in X/Open Curses Issue 7 for this utility,
unlike tput(1).
The -d (debug) and -n (no-op) options are extensions not provided by other implementations.
A tabs utility appeared in PWB/Unix 1.0 (1977). There was a reduced version of the tabs utility in Unix 7th edition and in 3BSD (1979). The latter supported a single ``-n'' option (to cause the first tab stop to be set on the left margin). That option is not documented by POSIX.
The PWB/Unix tabs utility, which was included in System III (1980), used built-in tables rather than the terminal database, to support a half-dozen terminal types. It also had built-in logic to support the left-margin, as well as a feature for copying the tab settings from a file.
Later versions of Unix, e.g., SVr4, added support for the terminal database, but kept the tables, as a fallback. In an earlier development effort, the tab-stop initialization provided by tset (1982) and incorporated into tput uses the terminal database,
POSIX documents no limits on the number of tab stops. Documentation for other implementations states that there is a limit on the number of tab stops (e.g., 20 in PWB/Unix's tabs utility). While some terminals may not accept an arbitrary number of tab stops, this implementation will attempt to set tab stops up to the right margin of the screen, if the given list happens to be that long.
The Rationale section of the POSIX documentation goes into some detail about the ways the committee considered redesigning the tabs and tput utilities, without proposing an improved solution. It comments that
no known historical version of tabs supports the capability of setting arbitrary tab stops.
However, the Explicit Lists described in this manual page were implemented in PWB/Unix. Those provide the capability of setting abitrary tab stops.
This describes ncurses version 6.2 (patch 20200222).