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Predefined registers whose identifiers start with a dot are read-only.
Many are Boolean-valued, interpolating a true or false value testable
with the if, ie, or while requests. Some read-only
registers are string-valued, meaning that they interpolate text.
Caution: Built-in registers are subject to removal like others; once removed, they can be recreated only as normal writable registers and will not reflect formatter state.
A register name (without the dot) is often associated with a request of the same name. A complete listing of all built-in registers can be found in Register Index.
We present here a few built-in registers that are not described
elsewhere in this manual; they have to do with invariant properties of
GNU troff, or obtain information about the formatter’s
command-line options, processing progress, or the operating environment.
\n[.A]Approximate output is being formatted (Boolean-valued); see
groff -a option (Groff Options).
\n[.c]\n[c.]Input line number. ‘c.’ is a writable synonym, affecting subsequent interpolations of both ‘.c’ and ‘c.’.
\n[.F]Name of input file (string-valued).
\n[.g]Always true in GNU troff (Boolean-valued). Documents can use
this to ask the formatter if it claims groff compatibility.
\n[.P]Output page selection status (Boolean-valued); see groff
-o option (Groff Options).
\n[.R]Count of available unused registers; always 10,000 in GNU
troff.52
\n[.T]Indicator of output device selection (Boolean-valued); see
groff -T option (Groff Options).
\n[.U]Unsafe mode enablement status (Boolean-valued); see groff
-U option (Groff Options).
\n[.x]Major version number of the running GNU troff formatter. For
example, if the version number is 1.23.0, then .x
contains ‘1’.
\n[.y]Minor version number of the running GNU troff formatter. For
example, if the version number is 1.23.0, then .y
contains ‘23’.
\n[.Y]Revision number of the running GNU troff formatter. For example,
if the version number is 1.23.0, then .Y contains ‘0’.
\n[$$]Process identifier (PID) of the GNU troff program in its
operating environment.
Date- and time-related registers are set per the local time as
determined by localtime(3) when the formatter launches. This
initialization can be overridden by SOURCE_DATE_EPOCH and
TZ; see Environment.
\n[seconds]Count of seconds elapsed in the minute (0–60).
\n[minutes]Count of minutes elapsed in the hour (0–59).
\n[hours]Count of hours elapsed since midnight (0–23).
\n[dw]Day of the week (1–7; 1 is Sunday).
\n[dy]Day of the month (1–31).
\n[mo]Month of the year (1–12).
\n[year]Gregorian year.
\n[yr]Gregorian year minus 1900. This register is incorrectly documented
in the AT&T troff manual as storing the last two digits
of the current year. That claim stopped being true in 2000. Old
troff input that looks like:
'\" The year number is a surprise after 1999. This document was formatted in 19\n(yr.
can be corrected to:
This document was formatted in \n[year].
or, for portability across many roff programs, to the following.
.nr y4 1900+\n(yr This document was formatted in \n(y4.
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