/
units.texinfo
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units.texinfo
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\input texinfo @c -*-texinfo-*-
@c %**start of header
@setfilename units.info
@settitle Units: A Unit Conversion Program
@finalout
@setchapternewpage off
@set EDITION 1.89
@set VERSION 1.87
@c %**end of header
@defcodeindex op
@syncodeindex op cp
@c noman
@dircategory Math
@ifinfo
@direntry
* units: (units). Units conversion.
@end direntry
This file documents the @sc{gnu} @code{units} command for unit conversion.
This is edition @value{EDITION} for @code{units} Version @value{VERSION}.
@c end noman
@c ifman .\"
Copyright (C) 1996, 1997, 1999, 2000, 2001, 2002, 2004, 2005, 2007 Free Software Foundation, Inc
This file is free software; the author gives unlimited permission to
copy, translate and/or distribute it, with or without modifications, as
long as this notice is preserved.
@c end ifman
@ignore
Permission is granted to process this file through Tex and print the
results, provided the printed document carries copying permission
notice identical to this one except for the removal of this paragraph
(this paragraph not being relevant to the printed manual).
@end ignore
@end ifinfo
@c man .TH UNITS 1 "25 Sep 2007"
@c man .SH NAME
@c man units - unit conversion program
@titlepage
@title Units Conversion
@subtitle Edition @value{EDITION} for @code{units} Version @value{VERSION}
@author Adrian Mariano
@page
@vskip 0pt plus 1filll
Copyright @copyright{} 1996, 1997, 1999, 2000, 2001, 2002, 2004, 2005,
2007 Free Software Foundation, Inc
The author gives unlimited permission to copy, translate and/or
distribute this document, with or without modifications, as long as this
notice is preserved.
@end titlepage
@iftex
@headings off
@everyheading Units Conversion @| @| @thispage
@end iftex
@node Top, , , (dir)
@c noman
@ifinfo
This manual describes the @code{units} command for units conversion, and
how you can use it to convert units from one scale to another.
This is Edition @value{EDITION} of @cite{The Units Conversion Manual}
for @code{units} Version @value{VERSION}.
@end ifinfo
@c end noman
@menu
* Overview:: What does @code{units} do?
* Interactive use:: How to use @code{units}.
* Command line use:: How to use @code{units} noninteractively.
* Invoking units:: Command line options.
* Unit expressions:: Forming compound units.
* Unit definitions:: What units are defined?
* Defining new units:: How to write your own units definitions.
* Nonlinear units:: How to define nonlinear units.
* Localization:: How to define and use regional unit names.
* Environment vars:: Environment variables used by @code{units}.
* Readline support:: Unit name completion and editing.
* Index:: General index
@end menu
@node Overview, Interactive use, Top, Top
@chapter Overview of @code{units}
The @code{units} program converts quantities expressed in various scales
to their equivalents in other scales. The @code{units} program can
handle multiplicative scale changes as well as nonlinear
conversions such as Fahrenheit to
@c man Celsius.
@c noman
Celsius.@footnote{But Fahrenheit to
Celsius is linear, you insist. Not so. A transformation @math{T} is linear if
@math{T(x+y)=T(x)+T(y)} and this fails for @math{T(x)=ax+b}. This transformation is
affine, but not linear. }
@c end noman
Temperature conversions require a special
syntax.
@c noman
@xref{Temperature Conversion Example}.
@c end noman
@c man See the examples below.
The units are defined in an external data file. You can use the
extensive data file that comes with this program, or you can
provide your own data file to suit your needs.
You can use the program interactively
with prompts, or you can use it
from the command line.
@node Interactive use, Command line use, Overview, Top
@chapter Interacting with @code{units}
@cindex interactive use
To invoke units for interactive use, type @kbd{units} at your shell
prompt. The program will print something like this:
@example
@group
2131 units, 53 prefixes, 24 nonlinear units
You have:
@end group
@end example
@noindent
At the @samp{You have:} prompt, type the quantity and units that
you are converting @emph{from}. For example, if you want to convert ten
meters to feet, type @kbd{10 meters}. Next, @code{units} will print
@samp{You want:}. You should type the type of units you want to convert
@emph{to}. To convert to feet, you would type @kbd{feet}. Note that if
the readline library was compiled in then the tab key can be used to
complete unit names. @xref{Readline support}, for more information
about readline.
The answer will be displayed in two ways. The first line of output,
which is marked with a @samp{*} to indicate multiplication,
gives the result of the conversion you have asked for. The second line
of output, which is marked with a @samp{/} to indicate division, gives
the inverse of the conversion factor. If you convert 10 meters to feet,
@code{units} will print
@example
@group
* 32.808399
/ 0.03048
@end group
@end example
@noindent
which tells you that 10 meters equals about 32.8 feet.
The second number gives the conversion in the opposite direction.
In this case, it tells you that 1 foot is equal to about
0.03 dekameters since the dekameter is 10 meters.
It also tells you that 1/32.8 is about .03.
The @code{units} program prints the inverse because sometimes it is a
more convenient number. In the example above, for example, the inverse
value is an exact conversion: a foot is exactly .03048 dekameters.
But the number given the other direction is inexact.
If you try to convert grains to pounds, you will see the following:
@example
@group
You have: grains
You want: pounds
* 0.00014285714
/ 7000
@end group
@end example
@noindent
From the second line of the output you can immediately see that a grain
is equal to a seven thousandth of a pound. This is not so obvious from
the first line of the output.
If you find the output format confusing, try using the
@samp{--verbose} option:
@cindex verbose output
@example
@group
You have: grain
You want: aeginamina
grain = 0.00010416667 aeginamina
grain = (1 / 9600) aeginamina
@end group
@end example
If you request a conversion between units which measure reciprocal
dimensions, then @code{units} will display the conversion results with an extra
note indicating that reciprocal conversion has been done:
@cindex reciprocal conversion
@example
@group
You have: 6 ohms
You want: siemens
reciprocal conversion
* 0.16666667
/ 6
@end group
@end example
Reciprocal conversion can be suppressed by using the @samp{--strict} option.
As usual, use
the @samp{--verbose} option to get more comprehensible output:
@cindex verbose output
@cindex strict conversion
@example
@group
You have: tex
You want: typp
reciprocal conversion
1 / tex = 496.05465 typp
1 / tex = (1 / 0.0020159069) typp
You have: 20 mph
You want: sec/mile
reciprocal conversion
1 / 20 mph = 180 sec/mile
1 / 20 mph = (1 / 0.0055555556) sec/mile
@end group
@end example
If you enter incompatible unit types, the @code{units} program will
print a message indicating that the units are not conformable and
it will display the reduced form for each unit:
@cindex incompatible units
@cindex non-conformable units
@example
@group
You have: ergs/hour
You want: fathoms kg^2 / day
conformability error
2.7777778e-11 kg m^2 / sec^3
2.1166667e-05 kg^2 m / sec
@end group
@end example
If you only want to find the reduced form or definition of a unit,
simply press return at the @samp{You want:} prompt. Here is an example:
@example
@group
You have: jansky
You want:
Definition: fluxunit = 1e-26 W/m^2 Hz = 1e-26 kg / s^2
@end group
@end example
@noindent
The output from @code{units} indicates that the jansky is defined to be
equal to a fluxunit which in turn is defined to be a certain combination
of watts, meters, and hertz. The fully reduced (and in this case
somewhat more cryptic) form appears on the far right.
Some named units are treated as dimensionless in some situations.
These include the radian and steradian. These units will be treated
as equal to 1 in units conversions. Power is equal to torque times
angular velocity. This conversion can only be performed if the radian
is dimensionless.
@example
@group
You have: (14 ft lbf) (12 radians/sec)
You want: watts
* 227.77742
/ 0.0043902509
@end group
@end example
@noindent
Note that named dimensionaless units are not treated as dimensionless
in other contexts. They cannot be used as exponents
so for example, @samp{meter^radian} is not allowed.
@cindex dimensionless units
If you want a list of options you can type @kbd{?} at the
@samp{You want:} prompt. The program will display a list of named
units which are conformable with the unit that you entered at
the @samp{You have:} prompt above. Note that conformable unit
combinations will not appear on this list.
Typing @kbd{help} at either prompt displays a short help message.
You can also type @kbd{help} followed by a unit name. This will
invoke a pager on the units data base at the point where that unit
is defined. You can read the definition and comments that may
give more details or historical information about the unit.
Typing @kbd{search text} will display a list of all of the units whose
names contain @samp{text} as a substring along with their definitions.
This may help in the case where you aren't sure of the right unit
name.
@node Command line use, Unit expressions, Interactive use, Top
@chapter Using @code{units} non-interactively
@cindex command line unit conversion
@cindex non-interactive unit conversion
The @code{units} program can perform units conversions non-interactively
from the command line. To do this, type the command, type the
original units expression, and type the new units you want.
You will probably need to protect the units expressions from
interpretation by the shell using single quote characters.
If you type
@example
units '2 liters' 'quarts'
@end example
@noindent
then @code{units} will print
@example
@group
* 2.1133764
/ 0.47317647
@end group
@end example
@noindent
and then exit.
The output tells you that 2 liters is about 2.1 quarts, or alternatively that
a quart is about 0.47 times 2 liters.
If the conversion is successful, then @code{units} will return success (0)
to the calling environment. If @code{units} is given non-conformable
units to convert, it will print a message giving the reduced form of
each unit and it will return failure (nonzero) to the calling environment.
When @code{units} is invoked with only one argument, it will print out
the definition of the specified unit. It will return failure if the
unit is not defined and success if the unit is defined.
@node Unit expressions, Invoking units, Command line use, Top
@comment node-name, next, previous, up
@chapter Unit expressions
@cindex unit expressions
In order to enter more complicated units or fractions,
you will need to use operations such as powers, products and division.
Powers of units can be specified using the @samp{^} character as shown in
the following example, or by simple concatenation: @samp{cm3} is equivalent to
@samp{cm^3}.
If the exponent is more than one digit, the @samp{^} is required. An
exponent like @samp{2^3^2} is evaluated right to left. The @samp{^}
operator has the second highest precedence. The @samp{**} operator is
provided as an alternative exponent operator.
@cindex powers
@cindex products
@example
@group
You have: cm^3
You want: gallons
* 0.00026417205
/ 3785.4118
You have: arabicfoot * arabictradepound * force
You want: ft lbf
* 0.7296
/ 1.370614
@end group
@end example
Multiplication of units can be specified by using spaces, or an asterisk
(@samp{*}). If @code{units} is invoked with the @samp{--product}
option then the hyphen (@samp{-}) also acts as a multiplication
operator. Division of units is indicated
by the slash (@samp{/}) or by @samp{per}.
@cindex multiplication of units
@cindex division of units
@example
@group
You have: furlongs per fortnight
You want: m/s
* 0.00016630986
/ 6012.8727
@end group
@end example
Historically, multiplication in units was assigned a higher precedence
than division. This disagrees with the usual precedence rules which
give multiplication and division equal precedence, and it has been a
source of confusion for people who think of units as a calculator.
By default, multiplication using the star (@samp{*}) now has the same
precedence as division and hence follows the usual precedence rules.
If units is invoked with the the @samp{--oldstar} option then then the
old behavior is activated and @samp{*} will have the same precedence
as the other multiplication operators described next.
Multiplication using a space or using the hyphen has a higher
precedence than division and is evaluated left to right. So @samp{m/s
s/day} is equivalent to @samp{m / s s day} and has dimensions of
length per time cubed. Similarly, @samp{1/2 meter} refers to a unit
of reciprocal length equivalent to .5/meter, which is probably not
what you would intend if you entered that expression.
You can indicate division of numbers with the vertical dash
(@samp{|}), so if you wanted half a meter you could write @samp{1|2
meter}. This operator has the highest precedence so the square root
of two thirds could be written @samp{2|3^1|2}.
@cindex fractions
@cindex division of numbers
@example
@group
You have: 1|2 inch
You want: cm
* 1.27
/ 0.78740157
@end group
@end example
@noindent
Parentheses can be used for grouping as desired.
@example
@group
You have: (1/2) kg / (kg/meter)
You want: league
* 0.00010356166
/ 9656.0833
@end group
@end example
Prefixes are defined separately from base units. In order to get
centimeters, the units database defines @samp{centi-} and @samp{c-} as
prefixes. Prefixes can appear alone with no unit following them.
An exponent applies only to the immediately preceding unit and its
prefix so that @samp{cm^3} or @samp{centimeter^3} refer to cubic centimeters
but @samp{centi*meter^3} refers to hundredths of cubic meters. Only one
prefix is permitted per unit, so @samp{micromicrofarad} will fail, but
@samp{micro*microfarad} will work, as will @samp{micro microfarad}..
For @code{units}, numbers are just another kind of unit. They can
appear as many times as you like and in any order in a unit expression.
For example, to find the volume of a box which is 2 ft by 3 ft by 12 ft
in steres, you could do the following:
@example
@group
You have: 2 ft 3 ft 12 ft
You want: stere
* 2.038813
/ 0.49048148
You have: $ 5 / yard
You want: cents / inch
* 13.888889
/ 0.072
@end group
@end example
@noindent
And the second example shows how the dollar sign in the units conversion
can precede the five. Be careful: @code{units} will interpret
@samp{$5} with no space as equivalent to dollars^5.
Outside of the SI system, it is often desirable to add values of
different units together. You may also wish to use @code{units} as a
calculator that keeps track of units. Sums of conformable units are written with
the @samp{+} character.
@cindex sums of units
@cindex addition of units
@example
@group
You have: 2 hours + 23 minutes + 32 seconds
You want: seconds
* 8612
/ 0.00011611705
@end group
@group
You have: 12 ft + 3 in
You want: cm
* 373.38
/ 0.0026782366
@end group
@group
You have: 2 btu + 450 ft lbf
You want: btu
* 2.5782804
/ 0.38785542
@end group
@end example
@noindent
The expressions which are added together must reduce to identical
expressions in primitive units, or an error message will be displayed:
@example
@group
You have: 12 printerspoint + 4 heredium
^
Illegal sum of non-conformable units
@end group
@end example
@noindent
Historically @samp{-} has been used for products of units, which complicates
its iterpretation in @code{units}. Because @code{units} provides
several other ways to obtain unit products, and because @samp{-} is a
subtraction operator in general algebraic expressions, @code{units}
treats the binary @samp{-} as a subtraction operator by default. This
behavior can be altered using the @samp{--product} option which
causes @code{units} to treat the binary @samp{-} operator as a product
operator. Note that when @samp{-} is a multiplication operator it has
the same precedence as @samp{*}, but when @samp{-} is a subtraction operator
it has the lower precedence as the addition operator.
When @samp{-} is used as a unary operator it negates its operand.
Regardless of the @code{units} options, if
@samp{-} appears after @samp{(} or after
@samp{+} then it will act as a negation operator. So you can always compute 20
degrees minus 12 minutes by entering @samp{20 degrees + -12 arcmin}.
You must use this construction when you define new units because you
cannot know what options will be in force when your definition is
processed.
The @samp{+} character sometimes appears in exponents like
@samp{3.43e+8}. This leads to an ambiguity in an expression like
@samp{3e+2 yC}. The unit @samp{e} is a small unit of charge, so this
can be regarded as equivalent to @samp{(3e+2) yC} or @samp{(3 e)+(2 yC)}.
This ambiguity is resolved by always interpreting @samp{+} as part
of an exponent if possible.
@cindex functions, built in
Several built in functions are provided: @samp{sin}, @samp{cos},
@samp{tan}, @samp{ln}, @samp{log}, @samp{log2}, @samp{exp}, @samp{acos},
@samp{atan} and @samp{asin}. The @samp{sin}, @samp{cos}, and @samp{tan}
functions require either a dimensionless argument or an argument with
dimensions of angle.
@example
@group
You have: sin(30 degrees)
You want:
Definition: 0.5
You have: sin(pi/2)
You want:
Definition: 1
You have: sin(3 kg)
^
Unit not dimensionless
@end group
@end example
@noindent
The other functions on the list require dimensionless arguments. The
inverse trigonometric functions return arguments with dimensions of
angle.
@cindex roots
@cindex square roots
If you wish to take roots of units, you may use the @samp{sqrt} or
@samp{cuberoot} functions. These functions require that the argument
have the appropriate root. Higher roots can be obtained by using
fractional exponents:
@example
@group
You have: sqrt(acre)
You want: feet
* 208.71074
/ 0.0047913202
You have: (400 W/m^2 / stefanboltzmann)^(1/4)
You have:
Definition: 289.80882 K
You have: cuberoot(hectare)
^
Unit not a root
@end group
@end example
@unnumberedsubsubsec Temperature Conversion Example
@cindex temperature conversions
@anchor{Temperature Conversion Example}
Nonlinear units are represented using functional notation. They make
possible nonlinear unit conversions such temperature. This is different
from the linear units that convert temperature differences. Note the
difference below. The absolute temperature conversions are handled by
units starting with @samp{temp}, and you must use functional notation.
The temperature differences are done using units starting with
@samp{deg} and they do not require functional notation.
@example
@group
You have: tempF(45)
You want: tempC
7.2222222
You have: 45 degF
You want: degC
* 25
/ 0.04
@end group
@end example
Think of @samp{tempF(x)} not as a function but as a notation which
indicates that @samp{x} should have units of @samp{tempF} attached to
it. @xref{Nonlinear units}. The first conversion shows that if it's 45
degrees Fahrehneit outside it's 7.2 degrees Celsius. The second
conversions indicates that a change of 45 degrees Fahrenheit corresponds
to a change of 25 degrees Celsius.
Some other examples of nonlinears units are ring size and wire gauge.
There are numerous different gauges and ring sizes. See the units
database for more details. Note that wire gauges
with multiple zeroes are signified using negative numbers where two
zeroes is -1. Alternatively, you can use the synonyms @samp{g00},
@samp{g000}, and so on that are defined in the units database.
@example
@group
You have: wiregauge(11)
You want: inches
* 0.090742002
/ 11.020255
You have: brwiregauge(g00)
You want: inches
* 0.348
/ 2.8735632
You have: 1 mm
You want: wiregauge
18.201919
@end group
@end example
@node Invoking units, Unit definitions, Unit expressions, Top
@chapter Invoking @code{units}
@cindex invoking units
@cindex command line options
You invoke @code{units} like this:
@example
units [@var{options}] [@var{from-unit} [@var{to-unit}]]
@end example
@noindent
If the @var{from-unit} and @var{to-unit} are omitted, then the program
will use interactive prompts to determine which conversions to perform.
@xref{Interactive use}.
If both @var{from-unit} and @var{to-unit} are given, @code{units} will
print the result of that single conversion and then exit.
If only @var{from-unit} appears on the command line, @code{units} will
display the definition of that unit and exit.
Units specified on the command line will need
to be quoted to protect them from shell interpretation and to group
them into two arguments. @xref{Command line use}.
The following options allow you to read in an alternative units file,
check your units file, or change the output format:
@table @samp
@item -c
@itemx --check
@opindex -c @r{(option for} @code{units}@r{)}
@opindex --check @r{(option for} @code{units}@r{)}
Check that all units and prefixes defined in the units data file reduce
to primitive units. Print a list of all units that
cannot be reduced. Also display some other diagnostics about
suspicious definitions in the units data file. Note that only
definitions active in the current locale are checked.
@item --check-verbose
@opindex --check-verbose @r{(option for} @code{units}@r{)}
@opindex --verbose-check @r{(option for} @code{units}@r{)}
Like the @samp{-check} option, this option prints a list of units that
cannot be reduced. But to help find unit definitions that cause
endless loops,
it lists the units as they are checked.
If @code{units} hangs, then the last unit to be printed has a bad
definition. Note that only
definitions active in the current locale are checked.
@item -o format
@itemx --output-format format
@opindex -o @r{(option for} @code{units}@r{)}
@opindex --output-format @r{(option for} @code{units}@r{)}
Use the specified format for numeric output. Format is the same
as that for the printf function in the ANSI C standard.
For example, if you want more precision you might use @samp{-o %.15g}.
@item -f filename
@itemx --file filename
@opindex -f @r{(option for} @code{units}@r{)}
@opindex --file @r{(option for} @code{units}@r{)}
Instruct @code{units} to load the units file @code{filename}.
If @code{filename} is the empty string (@samp{-f ''})
then the default units file will
be loaded. This enables you to load the default file plus a personal
units file. Up to 25 units files may be specified on the command line.
This option overrides the @code{UNITSFILE} environment variable.
@item -h
@itemx --help
@opindex -h @r{(option for} @code{units}@r{)}
@opindex --help @r{(option for} @code{units}@r{)}
Print out a summary of the options for @code{units}.
@item -m
@itemx --minus
@opindex -m @r{(option for} @code{units}@r{)}
@opindex --minus @r{(option for} @code{units}@r{)}
Causes @samp{-} to be interpreted as a subtraction operator. This is
usually the default behavior.
@item -p
@itemx --product
@opindex -p @r{(option for} @code{units}@r{)}
@opindex --product @r{(option for} @code{units}@r{)}
Causes @samp{-} to be interpreted as a multiplication operator when it
has two operands. It will as a negation operator when it has only one
operand: @samp{(-3)}. Note that by default @samp{-} is treated as a
subtraction operator.
@itemx --oldstar
@opindex --oldstar @r{(option for} @code{units}@r{)}
Causes @samp{*} to have the old style precedence, higher than the
precedence of division so that @samp{1/2*3} will equal @samp{6}.
@itemx --newstar
@opindex --newstar @r{(option for} @code{units}@r{)}
Forces @samp{*} to have the new (default) precedence which follows
the usual rules of algebra: the precedence of @samp{*} is the same as
the precedence of @samp{/}, so that @samp{1/2*3} will equal @samp{3/2}.
@itemx --compact
@opindex --compact @r{(option for} @code{units}@r{)}
Give compact output featuring only the conversion factor. This turns
off the @samp{--verbose} option.
@item -q
@itemx --quiet
@itemx --silent
@opindex -q @r{(option for} @code{units}@r{)}
@opindex --quiet @r{(option for} @code{units}@r{)}
@opindex --silent @r{(option for} @code{units}@r{)}
Suppress prompting of the user for units and the display of statistics
about the number of units loaded.
@item -s
@itemx --strict
@opindex -s @r{(option for} @code{units}@r{)}
@opindex --strict @r{(option for} @code{units}@r{)}
Suppress conversion of units to their reciprocal units. For
example, @code{units} will normally convert hertz to seconds
because these units are reciprocals of each other. The strict option
requires that units be strictly conformable to perform a conversion, and
will give an error if you attempt to convert hertz to seconds.
@item -1
@itemx --one-line
@opindex -1 @r{(option for} @code{units}@r{)}
@opindex --one-line @r{(option for} @code{units}@r{)}
Give only one line of output (the forward conversion). Do not print
the reverse conversion. Note that if a reciprocal conversion is
performed then @code{units} will print still print the ``reciprocal
conversion'' line.
@item -t
@itemx --terse
@opindex -t @r{(option for} @code{units}@r{)}
@opindex --terse @r{(option for} @code{units}@r{)}
Give terse output when converting units. This option can be used when
calling @code{units} from another program so that the output is easy to
parse. This option has the combined
effect of these options: @samp{--strict} @samp{--quiet} @samp{--one-line}
@samp{--compact}.
@item -v
@itemx --verbose
@opindex -v @r{(option for} @code{units}@r{)}
@opindex --verbose @r{(option for} @code{units}@r{)}
Give slightly more verbose output when converting units. When combined
with the @samp{-c} option this gives the same effect as
@samp{--check-verbose}.
@item -V
@itemx --version
@opindex -V @r{(option for} @code{units}@r{)}
@opindex --version @r{(option for} @code{units}@r{)}
Print program version number, tell whether the readline library
has been included, and give the location of the default units
data file.
@end table
@node Unit definitions, Defining new units, Invoking units, Top
@chapter Unit definitions
@cindex unit definitions
The conversion information is read from a units data file which
is called @file{units.dat} and is probably located in
the @file{/usr/local/share} directory.
If you invoke @code{units} with the @samp{-V} option, it will print
the location of this file.
The default
file includes definitions for all familiar units, abbreviations and
metric prefixes. It also includes many obscure or archaic units.
Many constants of nature are defined, including these:
@c noman
@example
pi @r{ratio of circumference to diameter}
c @r{speed of light}
e @r{charge on an electron}
force @r{acceleration of gravity}
mole @r{Avogadro's number}
water @r{pressure per unit height of water}
Hg @r{pressure per unit height of mercury}
au @r{astronomical unit}
k @r{Boltzman's constant}
mu0 @r{permeability of vacuum}
epsilon0 @r{permitivity of vacuum}
G @r{Gravitational constant}
mach @r{speed of sound}
@end example
@noindent
@c end noman
@c man .RS
@c man .TS
@c man l l.
@c man pi ratio of circumference to diameter
@c man c speed of light
@c man e charge on an electron
@c man force acceleration of gravity
@c man mole Avogadro's number
@c man water pressure per unit height of water
@c man Hg pressure per unit height of mercury
@c man au astronomical unit
@c man k Boltzman's constant
@c man mu0 permeability of vacuum
@c man epsilon0 permitivity of vacuum
@c man G gravitational constant
@c man mach speed of sound
@c man .TE
@c man .RE
The database includes atomic masses for all of the elements and numerous
other constants. Also included are the densities of various ingredients
used in baking so that @samp{2 cups flour_sifted} can be converted
to @samp{grams}. This is not an exhaustive list. Consult the units
data file to see the complete list, or to see the definitions that are
used.
The unit @samp{pound} is a unit of mass. To get force, multiply by the
force conversion unit @samp{force} or use the shorthand @samp{lbf}.
(Note that @samp{g} is already taken as the standard abbreviation for
the gram.) The unit @samp{ounce} is also a unit of mass. The fluid
ounce is @samp{fluidounce} or @samp{floz}. British capacity units that
differ from their US counterparts, such as the British Imperial gallon,
are prefixed with @samp{br}. Currency is prefixed with its country
name: @samp{belgiumfranc}, @samp{britainpound}.
The US Survey foot,
yard, and mile can be obtained by using the @samp{US} prefix.
These units differ slightly from the international length units.
They were in general use until 1959, and are still used
for geographic surveys.
The acre is officially defined in terms of the US Survey foot.
If you want an acre
defined according to the international foot, use @samp{intacre}. The
difference between these units is
about 4 parts per million.
The British also used a slightly different length measure before 1959.
These can be obtained with the prefix @samp{UK}.
When searching for
a unit, if the specified string does not appear exactly as a unit
name, then the @code{units} program will try to remove a
trailing @samp{s} or a trailing @samp{es}. If that fails, @code{units}
will check for a prefix.
All of the standard metric prefixes are defined.
To find out what units and prefixes are available, read the standard
units data file.
@node Defining new units, Nonlinear units, Unit definitions, Top
@chapter Defining new units
@cindex defining units
@cindex units, redefinition of
@cindex changing units definitions
All of the units and prefixes that @code{units} can convert are defined
in the units data file. If you want to add your own units, you can
supply your own file. You can also add your own units definitions in
the @file{.units.dat} file in your home directory. If this file
exists it is read before the units data file. It will not be read if
any units files are specified on the command line.
A unit is specified on a single line by giving its name and an
equivalence. Comments start with a @samp{#} character, which can appear
anywhere in a line. The backslash character (@samp{\})
acts as a continuation
character if it appears as the last character on a line, making it
possible to spread definitions out over several lines if desired.
A file can be included by giving the command @samp{!include} followed by
the file's name. The file will be sought in the same directory as the
parent file unless a full path is given.
@cindex include files
Unit names must not contain any of the operator characters @samp{+},
@samp{-}, @samp{*}, @samp{/}, @samp{|}, @samp{^} or the parentheses.
They cannot begin with a digit or a decimal point (@samp{.}), nor can
they end with a digit (except for zero).
Be careful to define
new units in terms of old ones so that a reduction leads to the
primitive units, which are marked with @samp{!} characters.
Dimensionless units are indicated by using the string
@samp{!dimensionless} for the unit definition.
@cindex dimensionless units
When adding new units, be sure to use the @samp{-c} option to check that
the new units reduce properly.
If you create a loop in the units definitions, then @code{units} will
hang when invoked with the @samp{-c} options. You will need to
use the @samp{--check-verbose} option which prints out each unit as it
checks them. The program will still hang, but the last unit printed
will be the unit which caused the infinite loop.
If you define any units which contain
@samp{+} characters, carefully check them because the @samp{-c} option
will not catch non-conformable sums. Be careful with the @samp{-}
operator as well. When used as a binary operator, the @samp{-}
character can perform addition or multiplication
depending on the options used to invoke @code{units}.
To ensure consistent behavior use @samp{-} only as a unary negation
operator when writing units definitions. To multiply two units leave a
space or use the @samp{*} operator with care, recalling that it has
two possible precedence values and may require parentheses to ensure
consistent behavior. To compute the difference
of @samp{foo} and @samp{bar} write @samp{foo+(-bar)} or even @samp{foo+-bar}.
Here is an example of a short units file that defines some basic
units:
@c noman
@example
@group
m ! # The meter is a primitive unit
sec ! # The second is a primitive unit
rad !dimensionless # A dimensionless primitive unit
micro- 1e-6 # Define a prefix
minute 60 sec # A minute is 60 seconds
hour 60 min # An hour is 60 minutes
inch 0.0254 m # Inch defined in terms of meters
ft 12 inches # The foot defined in terms of inches
mile 5280 ft # And the mile
@end group
@end example
@c end noman
@c man .RS
@c man .TS
@c man l l l.
@c man m ! # The meter is a primitive unit
@c man sec ! # The second is a primitive unit
@c man rad !dimensionless # The second is a primitive unit
@c man micro- 1e-6 # Define a prefix
@c man minute 60 sec # A minute is 60 seconds
@c man hour 60 min # An hour is 60 minutes
@c man inch 0.0254 m # Inch defined in terms of meters
@c man ft 12 inches # The foot defined in terms of inches
@c man mile 5280 ft # And the mile
@c man .TE
@c man .RS
@noindent
A unit which ends with a @samp{-} character is a prefix. If a prefix
definition contains any @samp{/} characters, be sure they are protected