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Name | Description | Concepts | History | Using roff | |
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roff(7) Miscellaneous Information Manual roff(7)
roff - concepts and history of roff typesetting
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 foundational to
understanding any roff implementation, narrate the development
history of some roff systems, detail the command pipeline managed
by groff(1), survey the formatting language, suggest tips for
editing roff input, and recommend further reading materials.
roff input contains 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. Hyphenation is
the process of splitting a word so that it appears partially on
one line, followed by a hyphen to indicate to the reader that the
word has been broken, and its remainder on the next. Hyphenation
break 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 formatter flushes any pending output
line upon encountering the end of input.
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.
Tab stops are by default located every half inch measured from the
drawing position corresponding to the beginning of the input line;
see subsection “Page geometry” below. 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 its origin near
the upper left corner. Coordinate values are in basic units and
increase down and to the right. Useful ones are typically
positive and within numeric ranges corresponding to the page
boundaries.
Text is arranged on a one-dimensional lattice of text baselines
from the top to the bottom of the page. A text baseline is a
(usually invisible) line upon which the glyphs of a typeface are
aligned. 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.
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.) A glyph therefore “starts” at its bottom-
left corner. The formatter's origin is thus one vee below the
page top, preventing a glyph from lying partially or wholly off
the page.
Further, it is conventional not to write or draw at the extreme
edges of the page. Typesetters configure a page offset, a
rightward shift from the left edge. (groff's terminal output
devices have page offsets of zero.)
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 of
at least 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 (given arguments) 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. Each register can be assigned a format, causing its
value to interpolate with leading zeroes, in Roman numerals, or
alphabetically. Some read-only registers are string-valued,
meaning that they interpolate text and lack a format.
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: fulfilling the condition springs the
trap—calls the macro.
Footnote support often exercises all three of the foregoing
features. A simple implementation might work as follows. The
author writes a pair of macros: one starts a footnote and the
other ends it. They further set a trap a small distance above the
page bottom, reserving a footnote area. 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.
It further creates an environment for the footnote so that it sets
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 body text nearly fills the page, the trap springs.
The macro called by the trap draws a line across the page and
emits the stored diversion. Thus, the footnote is rendered.
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.
[1mThe 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.
[1mUnix 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.
[1mNew 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 troff(1) man page in Fourth
Edition Unix (November 1973) does not feature a request list,
unlike nroff(1). 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 (1975) 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.
[1mgroff[24m—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 Bell
Labs device-independent troff), 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. Gunnar Ritter and later Carsten Kunze then
enhanced it 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.
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 mmroff(1), or more
general. groff(1) 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. (Non-Unix
systems may simulate pipelines 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 handling.
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 employs the macros of a document
formatting package, intermixed with instructions for one or more
preprocessors, and is 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; groff(1) 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 groff(1) 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
troff(1).
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 popular
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.
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 implementation of a
package name is 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 -m; see troff(1). It
can alternatively be specified within a document using the mso
request of the groff language; see groff(7).
Well-known packages include man for traditional man pages and mdoc
for BSD-style manual pages. 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.
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.
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.
• 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.
• 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.
• Use \& after “!”, “?”, and “.” if they are followed by space,
tab, or newline characters and don't end a sentence.
• In filled text lines, use \& before “.” and “'” if they are
preceded by space, so that revisions to the input don't turn
them into control lines.
• Do not use spaces to perform indentation or align columns of a
table. Leading spaces are reliable when text is not being
filled.
• 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.
• Use the empty request—a control character followed immediately
by a newline—to visually manage separation of material in the
input. 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.
• The most general approach includes 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:
• Certain file name extensions, like those often used by man
pages, activate nroff mode automatically.
• Loading a file with the sequence
.\" -*- nroff -*-
in its first line into an Emacs buffer causes the editor to
enter its nroff major mode. 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.
This document was written by Bernd Warken ⟨groff-bernd.warken-72@
web.de⟩ and G. Branden Robinson ⟨g.branden.robinson@gmail.com⟩.
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 those describing the language and its
device-independent implementation.
“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
A componentized system like roff potentially has many man pages,
each describing an aspect of it. Unfortunately, there is no
consistent naming scheme for these pages among the various
implementations.
In GNU roff, the groff(1) man page enumerates all man pages
distributed with the system, and individual pages frequently refer
to external resources as well as manuals on a variety of topics
imbricated with groff.
In other roffs, you are on your own, but troff(1) might be a good
starting point.
This page is part of the groff (GNU troff) project. Information
about the project can be found at
⟨http://www.gnu.org/software/groff/⟩. If you have a bug report for
this manual page, see ⟨http://www.gnu.org/software/groff/⟩. This
page was obtained from the project's upstream Git repository
⟨https://git.savannah.gnu.org/git/groff.git⟩ on 2025-02-02. (At
that time, the date of the most recent commit that was found in
the repository was 2025-01-28.) If you discover any rendering
problems in this HTML version of the page, or you believe there is
a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
groff 1.23.0.2722-658f-dirty 2025-01-02 roff(7)
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