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
+ 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.,
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 chang-
- ing the tabstops), and later cat the file to the screen, setting tab-
- stops at that point.
+ 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
+
+ tabs -8
+
+ or use the reset program, since the normal initialization sequences do
+ not ensure that tab-stops are reset.
-Tname
- Tell tabs which terminal type to use. If this option is not
- given, tabs will use the $TERM environment variable. If that is
+ Tell tabs which terminal type to use. If this option is not
+ given, tabs will use the $TERM environment variable. If that is
not set, it will use the ansi+tabs entry.
- -d The debugging option shows a ruler line, followed by two data
- lines. The first data line shows the expected tab-stops marked
- with asterisks. The second data line shows the actual tab-stops,
+ -d The debugging option shows a ruler line, followed by two data
+ lines. The first data line shows the expected tab-stops marked
+ with asterisks. The second data line shows the actual tab-stops,
marked with asterisks.
- -n This option tells tabs to check the options and run any debugging
+ -n This option tells tabs to check the options and run any debugging
option, but not to modify the terminal settings.
- -V reports the version of ncurses which was used in this program, and
+ -V reports the version of ncurses which was used in this program, and
exits.
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
+ to be processed which defines a list is the one that determines the
list to be processed.
- Use a single number as an option, e.g., "-5" to set tabs at the given
- interval (in this case 1, 6, 11, 16, 21, etc.). Tabs are repeated up
+ Use a single number as an option, e.g., "-5" to set tabs at the given
+ interval (in this case 1, 6, 11, 16, 21, etc.). Tabs are repeated up
to the right margin of the screen.
Use "-0" to clear all tabs.
@@ -102,15 +116,15 @@
- An explicit list can be defined after the options (this does not use a
- "-"). The values in the list must be in increasing numeric order, and
- greater than zero. They are separated by a comma or a blank, for exam-
- ple,
+ An explicit list can be defined after the options (this does not use a
+ "-"). The values in the list must be in increasing numeric order, and
+ greater than zero. They are separated by a comma or a blank, for
+ example,
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
+ Use a "+" to treat a number as an increment relative to the previous
value, e.g.,
tabs 1,+5,+5,+5,+5
@@ -118,57 +132,133 @@
which is equivalent to the 1,6,11,16,21 example.
-
+ POSIX defines several predefined lists of tab stops.
-a Assembler, IBM S/370, first format
+ 1,10,16,36,72
-a2 Assembler, IBM S/370, second format
+ 1,10,16,40,72
-c COBOL, normal format
+ 1,8,12,16,20,55
-c2 COBOL compact format
+ 1,6,10,14,49
-c3 COBOL compact format extended
+ 1,6,10,14,18,22,26,30,34,38,42,46,50,54,58,62,67
-f FORTRAN
+ 1,7,11,15,19,23
-p PL/I
+ 1,5,9,13,17,21,25,29,33,37,41,45,49,53,57,61
-s SNOBOL
+ 1,10,55
-u UNIVAC 1100 Assembler
+ 1,12,20,44
+
+
+
+ A few terminals expose a means of changing their left and right
+ margins. tabs supports this feature with an option.
+
+ +mmargin
+ The effect depends on whether the terminal has the margin
+ capabilities:
+
+ o If the terminal provides the capability for setting the left
+ margin, tabs uses this, and adjusts the available tab stop
+ widths.
+
+ o If the terminal does not provide the margin capabilities, tabs
+ imitates their effect, putting tab stops at appropriate places
+ on each line. The terminal's left margin is not modified.
+
+ If the margin parameter is omitted, the default is 10. Use +m0 to
+ reset the left margin, that is, to make it the left edge of the
+ terminal's display. Before setting a left margin, tabs resets the
+ margin to reduce problems that might arise from moving the cursor
+ to the left of the current left margin.
+
+ When setting or resetting the left margin, tabs may also reset the
+ right margin.
+
+
+
- IEEEStd1003.1/TheOpenGroupBaseSpecificationsIssue7
- (POSIX.1-2008) describes a tabs utility. However
+ IEEE Std 1003.1/The Open Group Base Specifications Issue 7
+ (POSIX.1-2008) describes a tabs utility. However,
- o This standard describes a +m option, to set a terminal's left-mar-
- gin. Very few of the entries in the terminal database provide this
- capability.
+ o this standard describes a +m option to set a terminal's left
+ margin. Very few of the entries in the terminal database provide
+ the set_left_margin (smgl) or set_left_margin_parm (smglp)
+ capabilities needed to support the feature.
- o There is no counterpart in X/Open Curses Issue 7 for this utility,
- unlike tput(1).
+ o 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.
+ The -d (debug) and -n (no-op) options are ncurses extensions not
+ provided by other implementations.
- Documentation for other implementations states that there is a limit on
- the number of tab stops. While some terminals may not accept an arbi-
- trary 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.
+
+ A tabs utility appeared in PWB/Unix 1.0 (1977). A reduced version
+ shipped in Seventh Edition Unix (early 1979) and in 3BSD (later the
+ same year); it supported a "-n" option to set the first tab stop at the
+ left margin. That option is not specified by POSIX.
-
- tset(1), infocmp(1m), curses(3x), terminfo(5).
+ The PWB/Unix tabs utility returned in System III (1980), and used
+ built-in tables to support a half-dozen hardcopy terminal (printer)
+ types. It also had logic to support setting the left margin, as well
+ as a feature for copying the tab settings from a file.
+
+ Versions of the program in later releases of AT&T Unix, such as SVr4,
+ added support for the terminal database, but retained the tables to
+ support the printers. By this time, System V tput had incorporated the
+ tab stop initialization feature of BSD's tset from 1982, but employed
+ the terminfo database to do so.
+
+ The +m option was documented in the POSIX Base Specifications Issue 5
+ (Unix98, 1997), then omitted in Issue 6 (Unix03, 2004) without express
+ motivation, though an introductory comment "and optionally adjusts the
+ margin" remains, overlooked in the removal. The tabs utility
+ documented in Issues 6 and later has no mechanism for setting margins.
+ The +m option in ncursestabs differs from the SVr4 feature by using
+ terminal capabilities rather than built-in tables.
+
+ POSIX documents no limit on the number of tab stops. Other
+ implementations impose one; the limit is 20 in PWB/Unix's tabs utility.
+ While some terminals may not accept an arbitrary number of tab stops,
+ ncursestabs attempts to set tab stops up to the right margin if the
+ list thereof is sufficiently long.
- This describes ncurses version 6.1 (patch 20190121).
+ The "Rationale" section of the Issue 6 tabs reference page details how
+ the committee considered redesigning the tabs and tput utilities,
+ without settling on an improved solution. It claims that
+
+ "no known historical version of tabs supports the capability of
+ setting arbitrary tab stops."
+
+ The feature described in subsection "Explicit Lists" above was
+ implemented in PWB/Unix, and permitted the setting of abitrary tab
+ stops nevertheless.
+
+
+