roff(7) Miscellaneous Information Manual roff(7)
Name
roff - concepts and history of roff typesetting
Description
The term roff denotes a family of document formatting systems known by
names like troff, nroff, and ditroff. A roff system consists of an
interpreter for an extensible text formatting language and a set of
programs for preparing output for various devices and file formats.
Unix-like operating systems often distribute a roff system. The manual
pages on Unix systems ("man pages") and bestselling books on software
engineering, including Brian Kernighan and Dennis Ritchie's The C
Programming Language and W. Richard Stevens's Advanced Programming in
the Unix Environment have been written using roff systems. GNU
roff--groff--is arguably the most widespread roff implementation.
Below we present typographical concepts that form the background of all
roff implementations, narrate the development history of some roff
systems, detail the command pipeline managed by roff(7), survey the
formatting language, suggest tips for editing roff input, and recommend
further reading materials.
Concepts
roff input files contain text interspersed with instructions to control
the formatter. Even in the absence of such instructions, a roff
formatter still processes its input in several ways, by filling,
hyphenating, breaking, and adjusting it, and supplementing it with
inter-sentence space. These processes are basic to typesetting, and
can be controlled at the input document's discretion.
When a device-independent roff formatter starts up, it obtains
information about the device for which it is preparing output from the
latter's description file (see groff_font(5)). An essential property
is the length of the output line, such as "6.5 inches".
The formatter interprets plain text files employing the Unix line-
ending convention. It reads input a character at a time, collecting
words as it goes, and fits as many words together on an output line as
it can--this is known as filling. To a roff system, a word is any
sequence of one or more characters that aren't spaces or newlines. The
exceptions separate words.
A roff formatter attempts to detect boundaries between sentences, and
supplies additional inter-sentence space between them. It flags
certain characters (normally "!", "?", and ".") as potentially ending a
sentence. When the formatter encounters one of these end-of-sentence
characters at the end of an input line, or one of them is followed by
two (unescaped) spaces on the same input line, it appends an inter-word
space followed by an inter-sentence space in the output. The dummy
character escape sequence \& can be used after an end-of-sentence
character to defeat end-of-sentence detection on a per-instance basis.
Normally, the occurrence of a visible non-end-of-sentence character (as
opposed to a space or tab) immediately after an end-of-sentence
character cancels detection of the end of a sentence. However, several
characters are treated transparently after the occurrence of an end-of-
sentence character. That is, a roff does not cancel end-of-sentence
detection when it processes them. This is because such characters are
often used as footnote markers or to close quotations and
parentheticals. The default set is ", ', ), ], *, \[dg], \[dd], \[rq],
and \[cq]. The last four are examples of special characters, escape
sequences whose purpose is to obtain glyphs that are not easily typed
at the keyboard, or which have special meaning to the formatter (like
\).
When an output line is nearly full, it is uncommon for the next word
collected from the input to exactly fill it--typically, there is room
left over only for part of the next word. The process of splitting a
word so that it appears partially on one line (with a hyphen to
indicate to the reader that the word has been broken) with its
remainder on the next is hyphenation. Hyphenation points can be
manually specified; groff also uses a hyphenation algorithm and
language-specific pattern files to decide which words can be hyphenated
and where. Hyphenation does not always occur even when the hyphenation
rules for a word allow it; it can be disabled, and when not disabled
there are several parameters that can prevent it in certain
circumstances.
Once an output line is full, the next word (or remainder of a
hyphenated one) is placed on a different output line; this is called a
break. In this document and in roff discussions generally, a "break"
if not further qualified always refers to the termination of an output
line. When the formatter is filling text, it introduces breaks
automatically to keep output lines from exceeding the configured line
length. After an automatic break, a roff formatter adjusts the line if
applicable (see below), and then resumes collecting and filling text on
the next output line.
Sometimes, a line cannot be broken automatically. This usually does
not happen with natural language text unless the output line length has
been manipulated to be extremely short, but it can with specialized
text like program source code. groff provides a means of telling the
formatter where the line may be broken without hyphens. This is done
with the non-printing break point escape sequence \:.
There are several ways to cause a break at a predictable location. A
blank input line not only causes a break, but by default it also
outputs a one-line vertical space (effectively a blank output line).
Macro packages may discourage or disable this "blank line method" of
paragraphing in favor of their own macros. A line that begins with one
or more spaces causes a break. The spaces are output at the beginning
of the next line without being adjusted (see below). Again, macro
packages may provide other methods of producing indented paragraphs.
Trailing spaces on text lines (see below) are discarded. The end of
input causes a break.
After the formatter performs an automatic break, it may then adjust the
line, widening inter-word spaces until the text reaches the right
margin. Extra spaces between words are preserved. Leading and
trailing spaces are handled as noted above. Text can be aligned to the
left or right margin only, or centered, using requests.
A roff formatter translates horizontal tab characters, also called
simply "tabs", in the input into movements to the next tab stop. These
tab stops are by default located every half inch measured from the
current position on the input line. With them, simple tables can be
made. However, this method can be deceptive, as the appearance (and
width) of the text in an editor and the results from the formatter can
vary greatly, particularly when proportional typefaces are used. A tab
character does not cause a break and therefore does not interrupt
filling. The formatter provides facilities for sophisticated table
composition; there are many details to track when using the "tab" and
"field" low-level features, so most users turn to the tbl(1)
preprocessor to lay out tables.
Requests and macros
A request is an instruction to the formatter that occurs after a
control character, which is recognized at the beginning of an input
line. The regular control character is a dot ".". Its counterpart,
the no-break control character, a neutral apostrophe "'", suppresses
the break implied by some requests. These characters were chosen
because it is uncommon for lines of text in natural languages to begin
with them. If you require a formatted period or apostrophe (closing
single quotation mark) where the formatter is expecting a control
character, prefix the dot or neutral apostrophe with the dummy
character escape sequence, "\&".
An input line beginning with a control character is called a control
line. Every line of input that is not a control line is a text line.
Requests often take arguments, words (separated from the request name
and each other by spaces) that specify details of the action the
formatter is expected to perform. If a request is meaningless without
arguments, it is typically ignored. Of key importance are the requests
that define macros. Macros are invoked like requests, enabling the
request repertoire to be extended or overridden.
A macro can be thought of as an abbreviation you can define for a
collection of control and text lines. When the macro is called by
giving its name after a control character, it is replaced with what it
stands for. The process of textual replacement is known as
interpolation. Interpolations are handled as soon as they are
recognized, and once performed, a roff formatter scans the replacement
for further requests, macro calls, and escape sequences.
In roff systems, the "de" request defines a macro.
Page geometry
roff systems format text under certain assumptions about the size of
the output medium, or page. For the formatter to correctly break a
line it is filling, it must know the line length, which it derives from
the page width. For it to decide whether to write an output line to
the current page or wait until the next one, it must know the page
length. A device's resolution converts practical units like inches or
centimeters to basic units, a convenient length measure for the output
device or file format. The formatter and output driver use basic units
to reckon page measurements. The device description file defines its
resolution and page dimensions (see groff_font(5)).
A page is a two-dimensional structure upon which a roff system imposes
a rectangular coordinate system with its upper left corner as the
origin. Coordinate values are in basic units and increase down and to
the right. Useful ones are therefore always positive and within
numeric ranges corresponding to the page boundaries.
While the formatter (and, later, output driver) is processing a page,
it keeps track of its drawing position, which is the location at which
the next glyph will be written, from which the next motion will be
measured, or where a geometric object will commence rendering.
Notionally, glyphs are drawn from the text baseline upward and to the
right. (groff does not yet support right-to-left scripts.) The text
baseline is a (usually invisible) line upon which the glyphs of a
typeface are aligned. A glyph therefore "starts" at its bottom-left
corner. If drawn at the origin, a typical letter glyph would lie
partially or wholly off the page, depending on whether, like "g", it
features a descender below the baseline.
Such a situation is nearly always undesirable. It is furthermore
conventional not to write or draw at the extreme edges of the page.
Therefore the initial drawing position of a roff formatter is not at
the origin, but below and to the right of it. This rightward shift
from the left edge is known as the page offset. (groff's terminal
output devices have page offsets of zero.) The downward shift leaves
room for a text output line.
Text is arranged on a one-dimensional lattice of text baselines from
the top to the bottom of the page. Vertical spacing is the distance
between adjacent text baselines. Typographic tradition sets this
quantity to 120% of the type size. The initial vertical drawing
position is one unit of vertical spacing below the page top.
Typographers term this unit a vee.
Vertical spacing has an impact on page-breaking decisions. Generally,
when a break occurs, the formatter moves the drawing position to the
next text baseline automatically. If the formatter were already
writing to the last line that would fit on the page, advancing by one
vee would place the next text baseline off the page. Rather than let
that happen, roff formatters instruct the output driver to eject the
page, start a new one, and again set the drawing position to one vee
below the page top; this is a page break.
When the last line of input text corresponds to the last output line
that fits on the page, the break caused by the end of input will also
break the page, producing a useless blank one. Macro packages keep
users from having to confront this difficulty by setting "traps";
moreover, all but the simplest page layouts tend to have headers and
footers, or at least bear vertical margins larger than one vee.
Other language elements
Escape sequences start with the escape character, a backslash \, and
are followed by at least one additional character. They can appear
anywhere in the input.
With requests, the escape and control characters can be changed;
further, escape sequence recognition can be turned off and back on.
Strings store character sequences. In groff, they can be parameterized
as macros can.
Registers store numerical values, including measurements. The latter
are generally in basic units; scaling units can be appended to numeric
expressions to clarify their meaning when stored or interpolated. Some
read-only predefined registers interpolate text.
Fonts are identified either by a name or by a mounting position (a non-
negative number). Four styles are available on all devices. R is
"roman": normal, upright text. B is bold, an upright typeface with a
heavier weight. I is italic, a face that is oblique on typesetter
output devices and usually underlined instead on terminal devices. BI
is bold-italic, combining both of the foregoing style variations.
Typesetting devices group these four styles into families of text
fonts; they also typically offer one or more special fonts that provide
unstyled glyphs; see groff_char(7).
groff supports named colors for glyph rendering and drawing of
geometric objects. Stroke and fill colors are distinct; the stroke
color is used for glyphs.
Glyphs are visual representation forms of characters. In groff, the
distinction between those two elements is not always obvious (and a
full discussion is beyond our scope). In brief, "A" is a character
when we consider it in the abstract: to make it a glyph, we must select
a typeface with which to render it, and determine its type size and
color. The formatting process turns input characters into output
glyphs. A few characters commonly seen on keyboards are treated
specially by the roff language and may not look correct in output if
used unthinkingly; they are the (double) quotation mark ("), the
neutral apostrophe ('), the minus sign (-), the backslash (\), the
caret or circumflex accent (^), the grave accent (`), and the tilde
(~). All of these and more can be produced with special character
escape sequences; see groff_char(7).
groff offers streams, identifiers for writable files, but for security
reasons this feature is disabled by default.
A further few language elements arise as page layouts become more
sophisticated and demanding. Environments collect formatting
parameters like line length and typeface. A diversion stores formatted
output for later use. A trap is a condition on the input or output,
tested automatically by the formatter, that is associated with a macro,
calling it when that condition is fulfilled.
Footnote support often exercises all three of the foregoing features.
A simple implementation might work as follows. A pair of macros is
defined: one starts a footnote and the other ends it. The author calls
the first macro where a footnote marker is desired. The macro
establishes a diversion so that the footnote text is collected at the
place in the body text where its corresponding marker appears. An
environment is created for the footnote so that it is set at a smaller
typeface. The footnote text is formatted in the diversion using that
environment, but it does not yet appear in the output. The document
author calls the footnote end macro, which returns to the previous
environment and ends the diversion. Later, after much more body text
in the document, a trap, set a small distance above the page bottom, is
sprung. The macro called by the trap draws a line across the page and
emits the stored diversion. Thus, the footnote is rendered.
History
Computer-driven document formatting dates back to the 1960s. The roff
system is intimately connected with Unix, but its origins lie with the
earlier operating systems CTSS, GECOS, and Multics.
The predecessor--RUNOFF
roff's ancestor RUNOFF was written in the MAD language by Jerry Saltzer
to prepare his Ph.D. thesis on the Compatible Time Sharing System
(CTSS), a project of the Massachusetts Institute of Technology (MIT).
This program is referred to in full capitals, both to distinguish it
from its many descendants, and because bits were expensive in those
days; five- and six-bit character encodings were still in widespread
usage, and mixed-case alphabetics in file names seen as a luxury.
RUNOFF introduced a syntax of inlining formatting directives amid
document text, by beginning a line with a period (an unlikely
occurrence in human-readable material) followed by a "control word".
Control words with obvious meaning like ".line length n" were supported
as well as an abbreviation system; the latter came to overwhelm the
former in popular usage and later derivatives of the program. A sample
of control words from a RUNOFF manual of December 1966 <http://web.mit
.edu/Saltzer/www/publications/ctss/AH.9.01.html> was documented as
follows (with the parameter notation slightly altered). The
abbreviations will be familiar to roff veterans.
Abbreviation Control word
.ad .adjust
.bp .begin page
.br .break
.ce .center
.in .indent n
.ll .line length n
.nf .nofill
.pl .paper length n
.sp .space [n]
In 1965, MIT's Project MAC teamed with Bell Telephone Laboratories and
General Electric (GE) to inaugurate the Multics <http://www.multicians
.org> project. After a few years, Bell Labs discontinued its
participation in Multics, famously prompting the development of Unix.
Meanwhile, Saltzer's RUNOFF proved influential, seeing many ports and
derivations elsewhere.
In 1969, Doug McIlroy wrote one such reimplementation, adding
extensions, in the BCPL language for a GE 645 running GECOS at the Bell
Labs location in Murray Hill, New Jersey. In its manual, the control
commands were termed "requests", their two-letter names were canonical,
and the control character was configurable with a .cc request. Other
familiar requests emerged at this time; no-adjust (.na), need (.ne),
page offset (.po), tab configuration (.ta, though it worked
differently), temporary indent (.ti), character translation (.tr), and
automatic underlining (.ul; on RUNOFF you had to backspace and
underscore in the input yourself). .fi to enable filling of output
lines got the name it retains to this day. McIlroy's program also
featured a heuristic system for automatically placing hyphenation
points, designed and implemented by Molly Wagner. It furthermore
introduced numeric variables, termed registers. By 1971, this program
had been ported to Multics and was known as roff, a name McIlroy
attributes to Bob Morris, to distinguish it from CTSS RUNOFF.
Unix and roff
McIlroy's roff was one of the first Unix programs. In Ritchie's term,
it was "transliterated" from BCPL to DEC PDP-7 assembly language for
the fledgling Unix operating system. Automatic hyphenation was managed
with .hc and .hy requests, line spacing control was generalized with
the .ls request, and what later roffs would call diversions were
available via "footnote" requests. This roff indirectly funded
operating systems research at Murray Hill; AT&T prepared patent
applications to the U.S. government with it. This arrangement enabled
the group to acquire a PDP-11; roff promptly proved equal to the task
of formatting the manual for what would become known as "First Edition
Unix", dated November 1971.
Output from all of the foregoing programs was limited to line printers
and paper terminals such as the IBM 2471 (based on the Selectric line
of typewriters) and the Teletype Corporation Model 37. Proportionally
spaced type was unavailable.
New roff and Typesetter roff
The first years of Unix were spent in rapid evolution. The
practicalities of preparing standardized documents like patent
applications (and Unix manual pages), combined with McIlroy's
enthusiasm for macro languages, perhaps created an irresistible
pressure to make roff extensible. Joe Ossanna's nroff, literally a
"new roff", was the outlet for this pressure. By the time of Unix
Version 3 (February 1973)--and still in PDP-11 assembly language--it
sported a swath of features now considered essential to roff systems:
definition of macros (.de), diversion of text thither (.di), and
removal thereof (.rm); trap planting (.wh; "when") and relocation (.ch;
"change"); conditional processing (.if); and environments (.ev).
Incremental improvements included assignment of the next page number
(.pn); no-space mode (.ns) and restoration of vertical spacing (.rs);
the saving (.sv) and output (.os) of vertical space; specification of
replacement characters for tabs (.tc) and leaders (.lc); configuration
of the no-break control character (.c2); shorthand to disable automatic
hyphenation (.nh); a condensation of what were formerly six different
requests for configuration of page "titles" (headers and footers) into
one (.tl) with a length controlled separately from the line length
(.lt); automatic line numbering (.nm); interactive input (.rd), which
necessitated buffer-flushing (.fl), and was made convenient with early
program cessation (.ex); source file inclusion in its modern form (.so;
though RUNOFF had an ".append" control word for a similar purpose) and
early advance to the next file argument (.nx); ignorable content (.ig);
and programmable abort (.ab).
Third Edition Unix also brought the pipe(2) system call, the explosive
growth of a componentized system based around it, and a "filter model"
that remains perceptible today. Equally importantly, the Bell Labs
site in Murray Hill acquired a Graphic Systems C/A/T phototypesetter,
and with it came the necessity of expanding the capabilities of a roff
system to cope with a variety of proportionally spaced typefaces at
multiple sizes. Ossanna wrote a parallel implementation of nroff for
the C/A/T, dubbing it troff (for "typesetter roff"). Unfortunately,
surviving documentation does not illustrate what requests were
implemented at this time for C/A/T support; the roff(7) man page in
Fourth Edition Unix (November 1973) does not feature a request list,
unlike roff(7). Apart from typesetter-driven features, Unix Version 4
roffs added string definitions (.ds); made the escape character
configurable (.ec); and enabled the user to write diagnostics to the
standard error stream (.tm). Around 1974, empowered with multiple type
sizes, italics, and a symbol font specially commissioned by Bell Labs
from Graphic Systems, Kernighan and Lorinda Cherry implemented eqn for
typesetting mathematics. In the same year, for Fifth Edition Unix,
Ossanna combined and reimplemented the two roffs in C, using that
language's preprocessor to generate both from a single source tree.
Ossanna documented the syntax of the input language to the nroff and
troff programs in the "Troff User's Manual", first published in 1976,
with further revisions as late as 1992 by Kernighan. (The original
version was entitled "Nroff/Troff User's Manual", which may partially
explain why roff practitioners have tended to refer to it by its AT&T
document identifier, "CSTR #54".) Its final revision serves as the de
facto specification of AT&T troff, and all subsequent implementors of
roff systems have done so in its shadow.
A small and simple set of roff macros was first used for the manual
pages of Unix Version 4 and persisted for two further releases, but the
first macro package to be formally described and installed was ms by
Michael Lesk in Version 6. He also wrote a manual, "Typing Documents
on the Unix System", describing ms and basic nroff/troff usage,
updating it as the package accrued features. Sixth Edition
additionally saw the debut of the tbl preprocessor for formatting
tables, also by Lesk.
For Unix Version 7 (January 1979), McIlroy designed, implemented, and
documented the man macro package, introducing most of the macros
described in groff_man(7) today, and edited volume 1 of the Version 7
manual using it. Documents composed using ms featured in volume 2,
edited by Kernighan.
Meanwhile, troff proved popular even at Unix sites that lacked a C/A/T
device. Tom Ferrin of the University of California at San Francisco
combined it with Allen Hershey's popular vector fonts to produce
vtroff, which translated troff's output to the command language used by
Versatec and Benson-Varian plotters.
Ossanna had passed away unexpectedly in 1977, and after the release of
Version 7, with the C/A/T typesetter becoming supplanted by alternative
devices such as the Mergenthaler Linotron 202, Kernighan undertook a
revision and rewrite of troff to generalize its design. To implement
this revised architecture, he developed the font and device description
file formats and the page description language that remain in use
today. He described these novelties in the article "A Typesetter-
independent TROFF", last revised in 1982, and like the troff manual
itself, it is widely known by a shorthand, "CSTR #97".
Kernighan's innovations prepared troff well for the introduction of the
Adobe PostScript language in 1982 and a vibrant market in laser
printers with built-in interpreters for it. An output driver for
PostScript, dpost, was swiftly developed. However, AT&T's software
licensing practices kept Ossanna's troff, with its tight coupling to
the C/A/T's capabilities, in parallel distribution with device-
independent troff throughout the 1980s. Today, however, all actively
maintained troffs follow Kernighan's device-independent design.
groff--a free roff from GNU
The most important free roff project historically has been groff, the
GNU implementation of troff, developed by James Clark starting in 1989
and distributed under copyleft <http://www.gnu.org/copyleft> licenses,
ensuring to all the availability of source code and the freedom to
modify and redistribute it, properties unprecedented in roff systems to
that point. groff rapidly attracted contributors, and has served as a
replacement for almost all applications of AT&T troff (exceptions
include mv, a macro package for preparation of viewgraphs and slides,
and the ideal preprocessor, which produces diagrams from mathematical
constraints). Beyond that, it has added numerous features; see
groff_diff(7). Since its inception and for at least the following
three decades, it has been used by practically all GNU/Linux and BSD
operating systems.
groff continues to be developed, is available for almost all operating
systems in common use (along with several obscure ones), and is free.
These factors make groff the de facto roff standard today.
Other free roffs
In 2007, Caldera/SCO and Sun Microsystems, having acquired rights to
AT&T Documenter's Workbench (DWB) troff (a descendant of the Bell Labs
code), released it under a free but GPL-incompatible license. This
implementation <https://github.com/n-t-roff/DWB3.3> was made portable
to modern POSIX systems, and adopted and enhanced first by Gunnar
Ritter and then Carsten Kunze to produce Heirloom Doctools troff
<https://github.com/n-t-roff/heirloom-doctools>.
In July 2013, Ali Gholami Rudi announced neatroff <https://github.com/
aligrudi/neatroff>, a permissively licensed new implementation.
Another descendant of DWB troff is part of Plan 9 from User Space
<https://9fans.github.io/plan9port/>. Since 2021, this troff has been
available under permissive terms.
Using roff
When you read a man page, often a roff is the program rendering it.
Some roff implementations provide wrapper programs that make it easy to
use the roff system from the shell's command line. These can be
specific to a macro package, like roff(7), or more general. roff(7)
provides command-line options sparing the user from constructing the
long, order-dependent pipelines familiar to AT&T troff users. Further,
a heuristic program, grog(1), is available to infer from a document's
contents which groff arguments should be used to process it.
The roff pipeline
A typical roff document is prepared by running one or more processors
in series, followed by a a formatter program and then an output driver
(or "device postprocessor"). Commonly, these programs are structured
into a pipeline; that is, each is run in sequence such that the output
of one is taken as the input to the next, without passing through
secondary storage. (On non-Unix systems, pipelines may have to be
simulated with temporary files.)
$ preproc1 < input-file | preproc2 | ... | troff [option] ... \
| output-driver
Once all preprocessors have run, they deliver pure roff language input
to the formatter, which in turn generates a document in a page
description language that is then interpreted by a postprocessor for
viewing, printing, or further processing.
Each program interprets input in a language that is independent of the
others; some are purely descriptive, as with tbl(1) and roff output,
and some permit the definition of macros, as with eqn(1) and roff
input. Most roff input files employ the macros of a document
formatting package, intermixed with instructions for one or more
preprocessors, and seasoned with escape sequences and requests from the
roff language. Some documents are simpler still, since their
formatting packages discourage direct use of roff requests; man pages
are a prominent example. Many features of the roff language are seldom
needed by users; only authors of macro packages require a substantial
command of them.
Preprocessors
A roff preprocessor is a program that, directly or ultimately,
generates output in the roff language. Typically, each preprocessor
defines a language of its own that transforms its input into that for
roff or another preprocessor. As an example of the latter, chem
produces pic input. Preprocessors must consequently be run in an
appropriate order; roff(7) handles this automatically for all
preprocessors supplied by the GNU roff system.
Portions of the document written in preprocessor languages are usually
bracketed by tokens that look like roff macro calls. roff preprocessor
programs transform only the regions of the document intended for them.
When a preprocessor language is used by a document, its corresponding
program must process it before the input is seen by the formatter, or
incorrect rendering is almost guaranteed.
GNU roff provides several preprocessors, including eqn, grn, pic, tbl,
refer, and soelim. See roff(7) for a complete list. Other
preprocessors for roff systems are known.
dformat depicts data structures;
grap constructs statistical charts; and
ideal draws diagrams using a constraint-based language.
Formatter programs
A roff formatter transforms roff language input into a single file in a
page description language, described in groff_out(5), intended for
processing by a selected device. This page description language is
specialized in its parameters, but not its syntax, for the selected
device; the format is device-independent, but not device-agnostic. The
parameters the formatter uses to arrange the document are stored in
device and font description files; see groff_font(5).
AT&T Unix had two formatters--nroff for terminals, and troff for
typesetters. Often, the name troff is used loosely to refer to both.
When generalizing thus, groff documentation prefers the term "roff".
In GNU roff, the formatter program is always roff(7).
Devices and output drivers
To a roff system, a device is a hardware interface like a printer, a
text or graphical terminal, or a standardized file format that
unrelated software can interpret. An output driver is a program that
parses the output of troff and produces instructions specific to the
device or file format it supports. An output driver might support
multiple devices, particularly if they are similar.
The names of the devices and their driver programs are not
standardized. Technological fashions evolve; the devices used for
document preparation when AT&T troff was first written in the 1970s are
no longer used in production environments. Device capabilities have
tended to increase, improving resolution and font repertoire, and
adding color output and hyperlinking. Further, to reduce file size and
processing time, AT&T troff's page description language placed low
limits on the magnitudes of some quantities it could represent. Its
PostScript output driver, dpost(1), had a resolution of 720 units per
inch; groff's grops(1) uses 72,000.
roff programming
Documents using roff are normal text files interleaved with roff
formatting elements. The roff language is powerful enough to support
arbitrary computation and it supplies facilities that encourage
extension. The primary such facility is macro definition; with this
feature, macro packages have been developed that are tailored for
particular applications.
Macro packages
Macro packages can have a much smaller vocabulary than roff itself;
this trait combined with their domain-specific nature can make them
easy to acquire and master. The macro definitions of a package are
typically kept in a file called name.tmac (historically, tmac.name).
Find details on the naming and placement of macro packages in
groff_tmac(5).
A macro package anticipated for use in a document can be declared to
the formatter by the command-line option roff(7). It can
alternatively be specified within a document using the mso request of
the groff language; see roff(7).
Well-known macro packages include man for traditional man pages and
mdoc for BSD-style manual pages. Macro packages for typesetting books,
articles, and letters include ms (from "manuscript macros"), me (named
by a system administrator from the first name of its creator, Eric
Allman), mm (from "memorandum macros"), and mom, a punningly named
package exercising many groff extensions. See groff_tmac(5) for more.
The roff formatting language
The roff language provides requests, escape sequences, macro definition
facilities, string variables, registers for storage of numbers or
dimensions, and control of execution flow. The theoretically minded
will observe that a roff is not a mere markup language, but Turing-
complete. It has storage (registers), it can perform tests (as in
conditional expressions like "(\n[i] >= 1)"), its "if" and related
requests alter the flow of control, and macro definition permits
unbounded recursion.
Requests and escape sequences are instructions, predefined parts of the
language, that perform formatting operations, interpolate stored
material, or otherwise change the state of the parser. The user can
define their own request-like elements by composing together text,
requests, and escape sequences ad libitum. A document writer will not
(usually) note any difference in usage for requests or macros; both are
found on control lines. However, there is a distinction; requests take
either a fixed number of arguments (sometimes zero), silently ignoring
any excess, or consume the rest of the input line, whereas macros can
take a variable number of arguments. Since arguments are separated by
spaces, macros require a means of embedding a space in an argument; in
other words, of quoting it. This then demands a mechanism of embedding
the quoting character itself, in case it is needed literally in a macro
argument. AT&T troff had complex rules involving the placement and
repetition of the double quote to achieve both aims. groff cuts this
knot by supporting a special character escape sequence for the neutral
double quote, "\[dq]", which never performs quoting in the typesetting
language, but is simply a glyph, `"'.
Escape sequences start with a backslash, "\". They can appear almost
anywhere, even in the midst of text on a line, and implement various
features, including the insertion of special characters with "\(xx" or
"\[xxx]", break suppression at input line endings with "\c", font
changes with "\f", type size changes with "\s", in-line comments with
"\"", and many others.
Strings store text. They are populated with the ds request and
interpolated using the \* escape sequence.
Registers store numbers and measurements. A register can be set with
the request nr and its value can be retrieved by the escape sequence
\n.
File naming conventions
The structure or content of a file name, beyond its location in the
file system, is not significant to roff tools. roff documents
employing "full-service" macro packages (see groff_tmac(5)) tend to be
named with a suffix identifying the package; we thus see file names
ending in .man, .ms, .me, .mm, and .mom, for instance. When installed,
man pages tend to be named with the manual's section number as the
suffix. For example, the file name for this document is roff.7.
Practice for "raw" roff documents is less consistent; they are
sometimes seen with a .t suffix.
Input conventions
Since troff fills text automatically, it is common practice in the roff
language to avoid visual composition of text in input files: the
esthetic appeal of the formatted output is what matters. Therefore,
roff input should be arranged such that it is easy for authors and
maintainers to compose and develop the document, understand the syntax
of roff requests, macro calls, and preprocessor languages used, and
predict the behavior of the formatter. Several traditions have accrued
in service of these goals.
o Follow sentence endings in the input with newlines to ease their
recognition. It is frequently convenient to end text lines after
colons and semicolons as well, as these typically precede independent
clauses. Consider doing so after commas; they often occur in lists
that become easy to scan when itemized by line, or constitute
supplements to the sentence that are added, deleted, or updated to
clarify it. Parenthetical and quoted phrases are also good
candidates for placement on text lines by themselves.
o Set your text editor's line length to 72 characters or fewer; see the
subsections below. This limit, combined with the previous item of
advice, makes it less common that an input line will wrap in your
text editor, and thus will help you perceive excessively long
constructions in your text. Recall that natural languages originate
in speech, not writing, and that punctuation is correlated with
pauses for breathing and changes in prosody.
o Use \& after "!", "?", and "." if they are followed by space, tab, or
newline characters and don't end a sentence.
o In filled text lines, use \& before "." and "'" if they are preceded
by space, so that reflowing the input doesn't turn them into control
lines.
o Do not use spaces to perform indentation or align columns of a table.
Leading spaces are reliable when text is not being filled.
o Comment your document. It is never too soon to apply comments to
record information of use to future document maintainers (including
your future self). The \" escape sequence causes troff to ignore the
remainder of the input line.
o Use the empty request--a control character followed immediately by a
newline--to visually manage separation of material in input files.
Many of the groff project's own documents use an empty request
between sentences, after macro definitions, and where a break is
expected, and two empty requests between paragraphs or other requests
or macro calls that will introduce vertical space into the document.
You can combine the empty request with the comment escape sequence to
include whole-line comments in your document, and even "comment out"
sections of it.
An example sufficiently long to illustrate most of the above
suggestions in practice follows. An arrow -> indicates a tab
character.
.\" nroff this_file.roff | less
.\" groff -T ps this_file.roff > this_file.ps
->The theory of relativity is intimately connected with
the theory of space and time.
.
I shall therefore begin with a brief investigation of
the origin of our ideas of space and time,
although in doing so I know that I introduce a
controversial subject. \" remainder of paragraph elided
.
.
->The experiences of an individual appear to us arranged
in a series of events;
in this series the single events which we remember
appear to be ordered according to the criterion of
\[lq]earlier\[rq] and \[lq]later\[rq], \" punct swapped
which cannot be analysed further.
.
There exists,
therefore,
for the individual,
an I-time,
or subjective time.
.
This itself is not measurable.
.
I can,
indeed,
associate numbers with the events,
in such a way that the greater number is associated with
the later event than with an earlier one;
but the nature of this association may be quite
arbitrary.
.
This association I can define by means of a clock by
comparing the order of events furnished by the clock
with the order of a given series of events.
.
We understand by a clock something which provides a
series of events which can be counted,
and which has other properties of which we shall speak
later.
.\" Albert Einstein, _The Meaning of Relativity_, 1922
Editing with Emacs
Official GNU doctrine holds that the best program for editing a roff
document is Emacs; see emacs(1). It provides an nroff major mode that
is suitable for all kinds of roff dialects. This mode can be activated
by the following methods.
When editing a file within Emacs the mode can be changed by typing "M-x
nroff-mode", where M-x means to hold down the meta key (often labelled
"Alt") while pressing and releasing the "x" key.
It is also possible to have the mode automatically selected when a roff
file is loaded into the editor.
o The most general method is to include file-local variables at the end
of the file; we can also configure the fill column this way.
.\" Local Variables:
.\" fill-column: 72
.\" mode: nroff
.\" End:
o Certain file name extensions, such as those commonly used by man
pages, trigger the automatic activation of the nroff mode.
o Technically, having the sequence
.\" -*- nroff -*-
in the first line of a file will cause Emacs to enter the nroff major
mode when it is loaded into the buffer. Unfortunately, some
implementations of the man(1) program are confused by this practice,
so we discourage it.
Editing with Vim
Other editors provide support for roff-style files too, such as vim(1),
an extension of the vi(1) program. Vim's highlighting can be made to
recognize roff files by setting the filetype option in a Vim modeline.
For this feature to work, your copy of vim must be built with support
for, and configured to enable, several features; consult the editor's
online help topics "auto-setting", "filetype", and "syntax". Then put
the following at the end of your roff files, after any Emacs
configuration:
.\" vim: set filetype=groff textwidth=72:
Replace "groff" in the above with "nroff" if you want highlighting that
does not recognize many of the GNU extensions to roff, such as request,
register, and string names longer than two characters.
Authors
This document was written by Bernd Warken <groff-bernd.warken-72@web
.de> and G. Branden Robinson <g.branden.robinson@gmail.com>.
See also
Much roff documentation is available. The Bell Labs papers describing
AT&T troff remain available, and groff is documented comprehensively.
Internet sites
Unix Text Processing <https://github.com/larrykollar/
Unix-Text-Processing>, by Dale Dougherty and Tim O'Reilly, 1987, Hayden
Books. This well-regarded text brings the reader from a state of no
knowledge of Unix or text editing (if necessary) to sophisticated
computer-aided typesetting. It has been placed under a free software
license by its authors and updated by a team of groff contributors and
enthusiasts.
"History of Unix Manpages" <http://manpages.bsd.lv/history.html>, an
online article maintained by the mdocml project, provides an overview
of roff development from Saltzer's RUNOFF to 2008, with links to
original documentation and recollections of the authors and their
contemporaries.
troff.org <http://www.troff.org/>, Ralph Corderoy's troff site,
provides an overview and pointers to much historical roff information.
Multicians <http://www.multicians.org/>, a site by Multics enthusiasts,
contains a lot of information on the MIT projects CTSS and Multics,
including RUNOFF; it is especially useful for its glossary and the many
links to historical documents.
The Unix Archive <http://www.tuhs.org/Archive/>, curated by the Unix
Heritage Society, provides the source code and some binaries of
historical Unices (including the source code of some versions of troff
and its documentation) contributed by their copyright holders.
Jerry Saltzer's home page <http://web.mit.edu/Saltzer/www/publications/
pubs.html> stores some documents using the original RUNOFF formatting
language.
groff <http://www.gnu.org/software/groff>, GNU roff's web site,
provides convenient access to groff's source code repository, bug
tracker, and mailing lists (including archives and the subscription
interface).
Historical roff documentation
Many AT&T troff documents are available online, and can be found at
Ralph Corderoy's site (see above) or via Internet search.
Of foremost significance are two mentioned in section "History" above,
describing the language and its device-independent implementation,
respectively.
"Troff User's Manual" by Joseph F. Ossanna, 1976 (revised by Brian W.
Kernighan, 1992), AT&T Bell Laboratories Computing Science Technical
Report No. 54.
"A Typesetter-independent TROFF" by Brian W. Kernighan, 1982, AT&T Bell
Laboratories Computing Science Technical Report No. 97.
You can obtain many relevant Bell Labs papers in PDF from Bernd
Warken's "roff classical" GitHub repository <https://github.com/
bwarken/roff_classical.git>.
Manual pages
As a system of multiple components, a roff system potentially has many
man pages, each describing an aspect of it. Unfortunately, there is no
consistent naming scheme for these pages among the different roff
implementations.
For GNU roff, the roff(7) man page enumerates all man pages
distributed with the system, and individual pages frequently refer to
external resources as well as manuals distributed with groff on a
variety of topics.
With other roffs, you are on your own, but roff(7) might be a good
starting point.
groff 1.23.0 2 July 2023 roff(7)
groff 1.23.0 - Generated Fri Dec 22 16:35:07 CST 2023