
This manual is for GNU Units (version 2.00), which performs units
conversions and units calculations. A copy of the license is in
cluded in the section entitled ``GNU Free Documentation Li
UNITS(1) UNITS(1)
cense''.
NAME
units  unit conversion and calculation program
SYNOPSIS
'units' [options] [fromunit [tounit]]
DESCRIPTION
The 'units' program converts quantities expressed in various systems of measurement to
their equivalents in other systems of measurement. Like many similar programs, it can
handle multiplicative scale changes. It can also handle nonlinear conversions such as
Fahrenheit to Celsius. See the examples below. The program can also perform conversions
from and to sums of units, such as converting between meters and feet plus inches.
Beyond simple unit conversions, 'units' can be used as a generalpurpose scientific calcu
lator that keeps track of units in its calculations. You can form arbitrary complex math
ematical expressions of dimensions including sums, products, quotients, powers, and even
roots of dimensions. Thus you can ensure accuracy and dimensional consistency when work
ing with long expressions that involve many different units that may combine in complex
ways.
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 also use your own data file to supplement the standard data file.
Basic operation is simple: you enter the units that you want to convert from and the units
that you want to convert to. You can use the program interactively with prompts, or you
can use it from the command line.
INTERACTING WITH UNITS
To invoke units for interactive use, type 'units' at your shell prompt. The program will
print something like this:
Currency exchange rates from 04/23/12
2516 units, 85 prefixes, 65 nonlinear units
You have:
At the 'You have:' prompt, type the quantity and units that you are converting from. For
example, if you want to convert ten meters to feet, type '10 meters'. Next, 'units' will
print 'You want:'. You should type the units you want to convert to. To convert to feet,
you would type 'feet'. If the 'readline' library was compiled in then the tab key can be
used to complete unit names. See Readline Support for more information about 'readline'.
To quit the program press CtrlC or CtrlD under Unix. Under Windows press CtrlZ.
The answer will be displayed in two ways. The first line of output, which is marked with
a '*' to indicate multiplication, gives the result of the conversion you have asked for.
The second line of output, which is marked with a '/' to indicate division, gives the
inverse of the conversion factor. If you convert 10 meters to feet, 'units' will print
* 32.808399
/ 0.03048
which tells you that 10 meters equals about 32.8 feet. The second number gives the con
version 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 0.03.
The '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 0.03048 dekameters. But the number given the other direction is inexact.
If you convert grains to pounds, you will see the following:
You have: grains
You want: pounds
* 0.00014285714
/ 7000
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 'verbose' option:
You have: grain
You want: aeginamina
grain = 0.00010416667 aeginamina
grain = (1 / 9600) aeginamina
If you request a conversion between units that measure reciprocal dimensions, then 'units'
will display the conversion results with an extra note indicating that reciprocal conver
sion has been done:
You have: 6 ohms
You want: siemens
reciprocal conversion
* 0.16666667
/ 6
Reciprocal conversion can be suppressed by using the 'strict' option. As usual, use the
'verbose' option to get more comprehensible output:
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
If you enter incompatible unit types, the 'units' program will print a message indicating
that the units are not conformable and it will display the reduced form for each unit:
You have: ergs/hour
You want: fathoms kg^2 / day
conformability error
2.7777778e11 kg m^2 / sec^3
2.1166667e05 kg^2 m / sec
If you only want to find the reduced form or definition of a unit, simply press Enter at
the 'You want:' prompt. Here is an example:
You have: jansky
You want:
Definition: fluxunit = 1e26 W/m^2 Hz = 1e26 kg / s^2
The output from '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 units 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.
You have: (14 ft lbf) (12 radians/sec)
You want: watts
* 227.77742
/ 0.0043902509
Named dimensionless units are not treated as dimensionless in other contexts. They cannot
be used as exponents so for example, 'meter^radian' is not allowed.
If you want a list of options you can type '?' at the 'You want:' prompt. The program
will display a list of named units that are conformable with the unit that you entered at
the 'You have:' prompt above. Conformable unit combinations will not appear on this list.
Typing 'help' at either prompt displays a short help message. You can also type '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. (You can generally quit out of the page
by pressing 'q'.)
Typing 'search' text will display a list of all of the units whose names contain text as a
substring along with their definitions. This may help in the case where you aren't sure
of the right unit name.
USING UNITS NONINTERACTIVELY
The 'units' program can perform units conversions noninteractively from the command line.
To do this, type the command, type the original unit expression, and type the new units
you want. If a units expression contains nonalphanumeric characters, you may need to
protect it from interpretation by the shell using single or double quote characters.
If you type
units "2 liters" quarts
then 'units' will print
* 2.1133764
/ 0.47317647
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 'units' will return success (zero) to the calling
environment. If you enter nonconformable units then 'units' will print a message giving
the reduced form of each unit and it will return failure (nonzero) to the calling environ
ment.
When you invoke 'units' 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.
UNIT DEFINITIONS
The conversion information is read from a units data file that is called
'definitions.units' and is usually located in the '/usr/share/units' directory. If you
invoke 'units' with the '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:
pi ratio of circumference to diameter
c speed of light
e charge on an electron
force acceleration of gravity
mole Avogadro's number
water pressure per unit height of water
Hg pressure per unit height of mercury
au astronomical unit
k Boltzman's constant
mu0 permeability of vacuum
epsilon0 permittivity of vacuum
G Gravitational constant
mach speed of sound
The standard data file 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
'2 cups flour_sifted' can be converted to 'grams'. This is not an exhaustive list. Con
sult the units data file to see the complete list, or to see the definitions that are
used.
The 'pound' is a unit of mass. To get force, multiply by the force conversion unit
'force' or use the shorthand 'lbf'. (Note that 'g' is already taken as the standard
abbreviation for the gram.) The unit 'ounce' is also a unit of mass. The fluid ounce is
'fluidounce' or 'floz'. British capacity units that differ from their US counterparts,
such as the British Imperial gallon, are prefixed with 'br'. Currency is prefixed with
its country name: 'belgiumfranc', 'britainpound'.
When searching for a unit, if the specified string does not appear exactly as a unit name,
then the 'units' program will try to remove a trailing 's', 'es'. Next units will replace
a trailing 'ies' with 'y'. If that fails, 'units' will check for a prefix. The database
includes all of the standard metric prefixes. Only one prefix is permitted per unit, so
'micromicrofarad' will fail. However, prefixes can appear alone with no unit following
them, so 'micro*microfarad' will work, as will 'micro microfarad'.
To find out which units and prefixes are available, read the standard units data file,
which is extensively annotated.
English Customary Units
English customary units differ in various ways in different regions. In Britain a complex
system of volume measurements featured different gallons for different materials such as a
wine gallon and ale gallon that different by twenty percent. This complexity was swept
away in 1824 by a reform that created an entirely new gallon, the British Imperial gallon
defined as the volume occupied by ten pounds of water. Meanwhile in the USA the gallon is
derived from the 1707 Winchester wine gallon, which is 231 cubic inches. These gallons
differ by about twenty percent. By default if 'units' runs in the 'en_GB' locale you will
get the British volume measures. If it runs in the 'en_US' locale you will get the US
volume measures. In other locales the default values are the US definitions. If you wish
to force different definitions then set the environment variable 'UNITS_ENGLISH' to either
'US' or 'GB' to set the desired definitions independent of the locale.
Before 1959, the value of a yard (and other units of measure defined in terms of it) dif
fered slightly among Englishspeaking countries. In 1959, Australia, Canada, New Zealand,
the United Kingdom, the United States, and South Africa adopted the Canadian value of
1 yard = 0.9144 m (exactly), which was approximately halfway between the values used by
the UK and the US; it had the additional advantage of making 1 inch = 2.54 cm (exactly).
This new standard was termed the International Yard. Australia, Canada, and the UK then
defined all customary lengths in terms of the International Yard (Australia did not define
the furlong or rod); because many US land surveys were in terms of the pre1959 units, the
US continued to define customary surveyors' units (furlong, chain, rod, and link) in terms
of the previous value for the foot, which was termed the US survey foot. The US defined a
US survey mile as 5280 US survey feet, and defined a statute mile as a US survey mile.
The US values for these units differ from the international values by about 2 ppm.
The 'units' program uses the international values for these units; the US values can be
obtained by using either the 'US' or the 'survey' prefix. In either case, the simple
familiar relationships among the units are maintained, e.g., 1 'furlong' = 660 'ft', and 1
'USfurlong' = 660 'USft', though the metric equivalents differ slightly between the two
cases. The 'US' prefix or the 'survey' prefix can also be used to obtain the US survey
mile and the value of the US yard prior to 1959, e.g., 'USmile' or 'surveymile' (but not
'USsurveymile'). To get the US value of the statute mile, use either 'USstatutemile' or
'USmile'.
Except for distances that extend over hundreds of miles (such as in the US State Plane
Coordinate System), the differences in the miles are usually insignificant:
You have: 100 surveymile  100 mile
You want: inch
* 12.672025
/ 0.078913984
The pre1959 UK values for these units can be obtained with the prefix 'UK'.
In the US, the acre is officially defined in terms of the US survey foot, but 'units' uses
a definition based on the international foot. If you want the official US acre use
'USacre' and similarly use 'USacrefoot' for the official US version of that unit. The
difference between these units is about 4 parts per million.
UNIT EXPRESSIONS
Operators
You can enter more complicated units by combining units with operations such as powers,
multiplication, division, addition, subtraction, and parentheses for grouping. You can
use the customary symbols for these operators when 'units' is invoked with its default
options. Additionally, 'units' supports some extensions, including high priority multi
plication using a space, and a high priority numerical
division operator ('') that can simplify some expressions.
Powers of units can be specified using the '^' character as shown in the following exam
ple, or by simple concatenation of a unit and its exponent: 'cm3' is equivalent to 'cm^3';
if the exponent is more than one digit, the '^' is required. An exponent like '2^3^2' is
evaluated right to left as usual. The '^' operator has the second highest precedence.
You can also use '**' as an exponent operator.
You have: cm^3
You want: gallons
* 0.00026417205
/ 3785.4118
You have: arabicfoot * arabictradepound * force
You want: ft lbf
* 0.7296
/ 1.370614
You multiply units using a space or an asterisk ('*'). The example above shows both
forms. You can divide units using the slash ('/') or with 'per'.
You have: furlongs per fortnight
You want: m/s
* 0.00016630986
/ 6012.8727
When a unit includes a prefix, exponent operators apply to the combination, so
'centimeter^3' gives cubic centimeters. If you separate the prefix from the unit with any
multiplication operator, such as 'centi meter^3', then the prefix is treated as a separate
unit, so the exponent does not apply. The second example would be a hundredth of a cubic
meter, not a centimeter.
Multiplication using a space has a higher precedence than division using a slash and is
evaluated left to right; in effect, the first '/' character marks the beginning of the
denominator of a unit expression. This makes it simple to enter a quotient with several
terms in the denominator: 'W / m^2 Hz'. If you multiply with '*' then you must group the
terms in the denominator with parentheses: 'W / (m^2 * Hz)'.
The higher precedence of the space operator may not always be advantageous. For example,
'm/s s/day' is equivalent to 'm / s s day' and has dimensions of length per time cubed.
Similarly, '1/2 meter' refers to a unit of reciprocal length equivalent to 0.5/meter, per
haps not what you would intend if you entered that expression. The '*' operator is
convenient for multiplying a sequence of quotients. With the '*' operator, the example
above becomes 'm/s * s/day', which is equivalent to 'm/day'. Similarly, you could write
'1/2 * meter' to get half a meter. Alternatively, parentheses can be used for grouping:
you could write '(1/2) meter' to get half a meter. See Complicated Unit Expressions for
an illustration of the various options.
The 'units' program supports another option for numerical fractions. You can indicate
division of numbers with the vertical bar (''), so if you wanted half a meter you could
write '12 meter'. This operator has the highest precedence, so you can write the square
root of two thirds '23^12'. You cannot use the vertical bar to indicate division of
nonnumerical units (e.g., 'ms' results in an error message).
You have: 12 inch
You want: cm
* 1.27
/ 0.78740157
You can use parentheses for grouping:
You have: (1/2) kg / (kg/meter)
You want: league
* 0.00010356166
/ 9656.0833
Sums and Differences of Units
Outside of the SI, it is sometimes desirable to add values of different units. You may
also wish to use 'units' as a calculator that keeps track of units. Sums of conformable
units are written with the '+' character, and differences with the '' character.
You have: 2 hours + 23 minutes + 32 seconds
You want: seconds
* 8612
/ 0.00011611705
You have: 12 ft + 3 in
You want: cm
* 373.38
/ 0.0026782366
You have: 2 btu + 450 ft lbf
You want: btu
* 2.5782804
/ 0.38785542
The expressions that are added or subtracted must reduce to identical expressions in prim
itive units, or an error message will be displayed:
You have: 12 printerspoint  4 heredium
^
Illegal sum of nonconformable units
As usual, the precedence for '+' and '' is lower than that of the other operators. A
fractional quantity such as 2 1/2 cups can be given as '(2+12) cups'; the parentheses are
necessary because multiplication has higher precedence than addition. If you omit the
parentheses, 'units' attempts to add '2' and '12 cups', and you get an error message:
You have: 2+12 cups
^
Illegal sum or difference of nonconformable units
The expression could also be correctly written as '(2+1/2) cups'. If you write '2 12
cups' the space is interpreted as multiplication so the result is the same as '1 cup'.
The '+' and '' characters sometimes appears in exponents like '3.43e+8'. This leads to
an ambiguity in an expression like '3e+2 yC'. The unit 'e' is a small unit of charge, so
this can be regarded as equivalent to '(3e+2) yC' or '(3 e)+(2 yC)'. This ambiguity is
resolved by always interpreting '+' and '' as part of an exponent if possible.
Numbers as Units
For '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 that
is 2 ft by 3 ft by 12 ft in steres, you could do the following:
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
And the second example shows how the dollar sign in the units conversion can precede the
five. Be careful: 'units' will interpret '$5' with no space as equivalent to 'dollar^5'.
Builtin Functions
Several builtin functions are provided: 'sin', 'cos', 'tan', 'ln', 'log', 'log2', 'exp',
'acos', 'atan' and 'asin'. The 'sin', 'cos', and 'tan' functions require either a dimen
sionless argument or an argument with dimensions of angle.
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
The other functions on the list require dimensionless arguments. The inverse trigonomet
ric functions return arguments with dimensions of angle.
If you wish to take roots of units, you may use the 'sqrt' or 'cuberoot' functions. These
functions require that the argument have the appropriate root. You can obtain higher
roots by using fractional exponents:
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
Complicated Unit Expressions
The 'units' program is especially helpful in ensuring accuracy and dimensional consistency
when converting lengthy unit expressions. For example, one form of the DarcyWeisbach
fluidflow equation is
Delta P = (8 / pi)^2 (rho fLQ^2) / d^5,
where Delta P is the pressure drop, rho is the mass density, f is the (dimensionless)
friction factor, L is the length of the pipe, Q is the volumetric flow rate, and d is the
pipe diameter. It might be desired to have the equation in the form
Delta P = A1 rho fLQ^2 / d^5
that accepted the user's normal units; for typical units used in the US, the required con
version could be something like
You have: (8/pi^2)(lbm/ft^3)ft(ft^3/s)^2(1/in^5)
You want: psi
* 43.533969
/ 0.022970568
The parentheses allow individual terms in the expression to be entered naturally, as they
might be read from the formula. Alternatively, the multiplication could be done with the
'*' rather than a space; then parentheses are needed only around 'ft^3/s' because of its
exponent:
You have: 8/pi^2 * lbm/ft^3 * ft * (ft^3/s)^2 /in^5
You want: psi
* 43.533969
/ 0.022970568
Without parentheses, and using spaces for multiplication, the previous conversion would
need to be entered as
You have: 8 lb ft ft^3 ft^3 / pi^2 ft^3 s^2 in^5
You want: psi
* 43.533969
/ 0.022970568
Backwards Compatibility:
'*' and '' The original 'units' assigned multiplication a higher precedence than division
using the slash. This differs from the usual precedence rules, which give multiplication
and division equal precedence, and can be confusing for people who think of units as a
calculator.
The star operator ('*') included in this 'units' program has, by default, the same prece
dence as division, and hence follows the usual precedence rules. For backwards compati
bility you can invoke 'units' with the 'oldstar' option. Then '*' has a higher prece
dence than division, and the same precedence as multiplication using the space.
Historically, the hyphen ('') has been used in technical publications to indicate prod
ucts of units, and the original 'units' program treated it as a multiplication operator.
Because 'units' provides several other ways to obtain unit products, and because '' is a
subtraction operator in general algebraic expressions, 'units' treats the binary '' as a
subtraction operator by default. For backwards compatibility use the 'product' option,
which causes 'units' to treat the binary '' operator as a product operator. When '' is
a multiplication operator it has the same precedence as multiplication with a space, giv
ing it a higher precedence than division.
When '' is used as a unary operator it negates its operand. Regardless of the 'units'
options, if '' appears after '(' or after '+' then it will act as a negation operator.
So you can always compute 20 degrees minus 12 minutes by entering '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.
NONLINEAR UNIT CONVERSIONS
Nonlinear units are represented using functional notation. They make possible nonlinear
unit conversions such as temperature.
Temperature Conversions
Conversions between temperatures are different from linear conversions between temperature
incrementssee the example below. The absolute temperature conversions are handled by
units starting with 'temp', and you must use functional notation. The temperatureincre
ment conversions are done using units starting with 'deg' and they do not require func
tional notation.
You have: tempF(45)
You want: tempC
7.2222222
You have: 45 degF
You want: degC
* 25
/ 0.04
Think of 'tempF(x)' not as a function but as a notation that indicates that x should have
units of 'tempF' attached to it. See Defining Nonlinear Units. The first conversion
shows that if it's 45 degrees Fahrenheit outside, it's 7.2 degrees Celsius. The second
conversion indicates that a change of 45 degrees Fahrenheit corresponds to a change of 25
degrees Celsius. The conversion from 'tempF(x)' is to absolute temperature, so that
You have: tempF(45)
You want: degR
* 504.67
/ 0.0019814929
gives the same result as
You have: tempF(45)
You want: tempR
* 504.67
/ 0.0019814929
But if you convert 'tempF(x)' to 'degC', the output is probably not what you expect:
You have: tempF(45)
You want: degC
* 280.37222
/ 0.0035666871
The result is the temperature in K, because 'degC' is defined as 'K', the Kelvin. For con
sistent results, use the 'tempX' units when converting to a temperature rather than con
verting a temperature increment.
Other Nonlinear Units
Some other examples of nonlinear units are numerous different ring sizes and wire gauges,
the grit sizes used for abrasives, the decibel scale, shoe size, scales for the density of
sugar (e.g. baume). The standard data file also supplies units for computing the area of
a circle and the volume of a sphere. See the standard units data file for more details.
Wire gauges with multiple zeroes are signified using negative numbers where two zeroes is
'1'. Alternatively, you can use the synonyms 'g00', 'g000', and so on that are defined
in the standard units data file.
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
You have: grit_P(600)
You want: grit_ansicoated
342.76923
The last example shows the conversion from P graded sand paper, which is the European
standard and may be marked ``P600'' on the back, to the USA standard.
You can compute the area of a circle using the nonlinear unit, 'circlearea'. You can also
do this using the circularinch or circleinch. The next example shows two ways to compute
the area of a circle with a five inch radius and one way to compute the volume of a sphere
with a radius of one meter.
You have: circlearea(5 in)
You want: in2
* 78.539816
/ 0.012732395
You have: 10^2 circleinch
You want: in2
* 78.539816
/ 0.012732395
You have: spherevol(meter)
You want: ft3
* 147.92573
/ 0.0067601492
UNIT LISTS: CONVERSION TO SUMS OF UNITS
Outside of the SI, it is sometimes desirable to convert a single unit to a sum of units
for example, feet to feet plus inches. The conversion from sums of units was described in
Sums and Differences of Units and is a simple matter of adding the units with the '+'
sign:
You have: 12 ft + 3 in + 38 in
You want: ft
* 12.28125
/ 0.081424936
Although you can similarly write a sum of units to convert to, the result will not be the
conversion to the units in the sum, but rather the conversion to the particular sum that
you have entered:
You have: 12.28125 ft
You want: ft + in + 18 in
* 11.228571
/ 0.089058524
The unit expression given at the 'You want:' prompt is equivalent to asking for conversion
to multiples of '1 ft + 1 in + 18 in', which is 1.09375 ft, so the conversion in the pre
vious example is equivalent to
You have: 12.28125 ft
You want: 1.09375 ft
* 11.228571
/ 0.089058524
In converting to a sum of units like miles, feet and inches, you typically want the
largest integral value for the first unit, followed by the largest integral value for the
next, and the remainder converted to the last unit. You can do this conversion easily
with 'units' using a special syntax for lists of units. You must list the desired units
in order from largest to smallest, separated by the semicolon (';') character:
You have: 12.28125 ft
You want: ft;in;18 in
12 ft + 3 in + 38 in
The conversion always gives integer coefficients on the units in the list, except possibly
the last unit when the conversion is not exact:
You have: 12.28126 ft
You want: ft;in;18 in
12 ft + 3 in + 3.00096 * 18 in
The order in which you list the units is important:
You have: 3 kg
You want: oz;lb
105 oz + 0.051367866 lb
You have: 3 kg
You want: lb;oz
6 lb + 9.8218858 oz
Listing ounces before pounds produces a technically correct result, but not a very useful
one. You must list the units in descending order of size in order to get the most useful
result.
Ending a unit list with the separator ';' has the same effect as repeating the last unit
on the list, so 'ft;in;18 in;' is equivalent to 'ft;in;18 in;18 in'. With the example
above, this gives
You have: 12.28126 ft
You want: ft;in;18 in;
12 ft + 3 in + 38 in + 0.00096 * 18 in
in effect separating the integer and fractional parts of the coefficient for the last
unit. If you instead prefer to round the last coefficient to an integer you can do this
with the 'round' ('r') option. With the previous example, the result is
You have: 12.28126 ft
You want: ft;in;18 in
12 ft + 3 in + 38 in (rounded down to nearest 18 in)
When you use the 'r' option, repeating the last unit on the list has no effect (e.g.,
'ft;in;18 in;18 in' is equivalent to 'ft;in;18 in'), and hence neither does ending a
list with a ';'. With a single unit and the 'r' option, a terminal ';' does have an
effect: it causes 'units' to treat the single unit as a list and produce a rounded value
for the single unit. Without the extra ';', the 'r' option has no effect on single unit
conversions. This example shows the ouput using the 'r' option:
You have: 12.28126 ft
You want: in
* 147.37512
/ 0.0067854058
You have: 12.28126 ft
You want: in;
147 in (rounded down to nearest in)
Each unit that appears in the list must be conformable with the first unit on the list,
and of course the listed units must also be comformable with the You have unit that you
enter.
You have: meter
You want: ft;kg
^
conformability error
ft = 0.3048 m
kg = 1 kg
You have: meter
You want: lb;oz
conformability error
1 m
0.45359237 kg
In the first case, 'units' reports the disagreement between units appearing on the list.
In the second case, 'units' reports disagreement between the unit you entered and the
desired conversion. This conformability error is based on the first unit on the unit
list.
Other common candidates for conversion to sums of units are angles and time:
You have: 23.437754 deg
You want; deg;arcmin;arcsec
23 deg + 26 arcmin + 15.9144 arcsec
You have: 7.2319 hr
You want: hr;min;sec
7 hr + 13 min + 54.84 sec
In North America, recipes for cooking typically measure ingredients by volume, and use
units that are not always convenient multiples of each other. Suppose that you have a
recipe for 6 and you wish to make a portion for 1. If the recipe calls for 2 1/2 cups of
an ingredient, you might wish to know the measurements in terms of measuring devices you
have available, you could use 'units' and enter
You have: (2+12) cup / 6
You want: cup;12 cup;13 cup;14 cup;tbsp;tsp;12 tsp;14 tsp
13 cup + 1 tbsp + 1 tsp
By default, if a unit in a list begins with fraction of the form 1x and its multiplier is
an integer, the fraction is given as the product of the multiplier and the numerator; for
example,
You have: 12.28125 ft
You want: ft;in;18 in;
12 ft + 3 in + 38 in
In many cases, such as the example above, this is what is wanted, but sometimes it is not.
For example, a cooking recipe for 6 might call for 5 1/4 cup of an ingredient, but you
want a portion for 2, and your 1cup measure is not available; you might try
You have: (5+14) cup / 3
You want: 12 cup;13 cup;14 cup
32 cup + 14 cup
This result might be fine for a baker who has a 1 1/2cup measure (and recognizes the
equivalence), but it may not be as useful to someone with more limited set of measures,
who does want to do additional calculations, and only wants to know ``How many 1/2cup
measures to I need to add?'' After all, that's what was actually asked. With the
'showfactor' option, the factor will not be combined with a unity numerator, so that
you get
You have: (5+14) cup / 3
You want: 12 cup;13 cup;14 cup
3 * 12 cup + 14 cup
A userspecified fractional unit with a numerator other than 1 is never overridden, how
everif a unit list specifies '34 cup;12 cup', a result equivalent to 1 1/2 cups will
always be shown as '2 * 34 cup' whether or not the 'showfactor' option is given.
Some applications for unit lists may be less obvious. Suppose that you have a postal
scale and wish to ensure that it's accurate at 1 oz, but have only metric calibration
weights. You might try
You have: 1 oz
You want: 100 g;50 g; 20 g;10 g;5 g;2 g;1 g;
20 g + 5 g + 2 g + 1 g + 0.34952312 * 1 g
You might then place one each of the 20 g, 5 g, 2 g, and 1 g weights on the scale and hope
that it indicates close to
You have: 20 g + 5 g + 2 g + 1 g
You want: oz;
0.98767093 oz
Appending ';' to 'oz' forces a oneline display that includes the unit; here the integer
part of the result is zero, so it is not displayed.
A unit list such as
cup;12 cup;13 cup;14 cup;tbsp;tsp;12 tsp;14 tsp
can be tedious to enter. The 'units' program provides shorthand names for some common
combinations:
hms hours, minutes, seconds
dms angle: degrees, minutes, seconds
time years, days, hours, minutes and seconds
usvol US cooking volume: cups and smaller
Using these shorthands, or unit list aliases, you can do the following conversions:
You have: anomalisticyear
You want: time
1 year + 25 min + 3.4653216 sec
You have: 16 cup
You want: usvol
2 tbsp + 2 tsp
You cannot combine a unit list alias with other units: it must appear alone at the 'You
want:' prompt.
You can display the definition of a unit list alias by entering it at the 'You have:'
prompt:
You have: dms
Definition: unit list, deg;arcmin;arcsec
When you specify compact output with 'compact', 'terse' or 't' and perform conversion
to a unit list, 'units' lists the conversion factors for each unit in the list, separated
by semicolons.
You have: year
You want: day;min;sec
365;348;45.974678
Unlike the case of regular output, zeros are included in this output list:
You have: liter
You want: cup;12 cup;14 cup;tbsp
4;0;0;3.6280454
INVOKING UNITS
You invoke 'units' like this:
units [options] [fromunit [tounit]]
If the fromunit and tounit are omitted, then the program will use interactive prompts to
determine which conversions to perform. See Interactive Use. If both fromunit and to
unit are given, 'units' will print the result of that single conversion and then exit. If
only fromunit appears on the command line, 'units' will display the definition of that
unit and exit. Units specified on the command line may need to be quoted to protect them
from shell interpretation and to group them into two arguments. See Command Line Use.
The following options allow you to read in an alternative units file, check your units
file, or change the output format:
c, check
Check that all units and prefixes defined in the units data file reduce to primi
tive units. Print a list of all units that cannot be reduced. Also display some
other diagnostics about suspicious definitions in the units data file. Only defi
nitions active in the current locale are checked. You should always run 'units'
with this option after modifying a units data file.
checkverbose, verbosecheck
Like the '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 'units' hangs, then the last unit to be printed has
a bad definition. Only definitions active in the current locale are checked.
o format, outputformat format
Use the specified format for numeric output; the format is a subset of that for the
printf function in the ANSI C standard. Only a numeric format ('E' or 'e' for sci
entific notation, 'f' for fixedpoint decimal, or 'G' or 'g' to specify the number
of significant figures) is allowed. The default format is '%.8g'; for greater pre
cision, you could specify 'o %.15g'. See Numeric Output Format and the documenta
tion for printf() for more detailed descriptions of the format specification.
e, exponential
Set the numeric output format to exponential (i.e., scientific notation), like that
used in the Unix 'units' program.
f filename, file filename
Instruct 'units' to load the units file 'filename'. You can specify up to 25 units
files on the command line. When you use this option, 'units' will load only the
files you list on the command line; it will not load the standard file or your per
sonal units file unless you explicitly list them. If filename is the empty string
('f ""'), the default units file (or that specified by 'UNITSFILE') will be loaded
in addition to any others specified with 'f'.
h, help
Print out a summary of the options for 'units'.
m, minus
Causes '' to be interpreted as a subtraction operator. This is the default behav
ior.
p, product
Causes '' to be interpreted as a multiplication operator when it has two operands.
It will act as a negation operator when it has only one operand: '(3)'. By
default '' is treated as a subtraction operator.
oldstar
Causes '*' to have the oldstyle precedence, higher than the precedence of division
so that '1/2*3' will equal '1/6'.
newstar
Forces '*' to have the new (default) precedence that follows the usual rules of
algebra: the precedence of '*' is the same as the precedence of '/', so that
'1/2*3' will equal '3/2'.
compact
Give compact output featuring only the conversion factor. This turns off the
'verbose' option.
q, quiet, silent
Suppress prompting of the user for units and the display of statistics about the
number of units loaded.
n, nolists
Disable conversion to unit lists.
r, round
When converting to a combination of units given by a unit list, round the value of
the last unit in the list to the nearest integer.
S, showfactor
When converting to a combination of units specified in a list, always show a non
unity factor before a unit that begins with a fraction with a unity denominator.
By default, if the unit in a list begins with fraction of the form 1x and its mul
tiplier is an integer other than 1, the fraction is given as the product of the
multiplier and the numerator (e.g., '38 in' rather than '3 * 18 in'). In some
cases, this is not what is wanted; for example, the results for a cooking recipe
might show '3 * 12 cup' as '32 cup'. With the 'showfactor' option, a result
equivalent to 1.5 cups will display as '3 * 12 cup' rather than '32 cup'. A
userspecified fractional unit with a numerator other than 1 is never overridden,
howeverif a unit list specifies '34 cup;12 cup', a result equivalent to 1 1/2
cups will always be shown as '2 * 34 cup' whether or not the 'showfactor'
option is given.
s, strict
Suppress conversion of units to their reciprocal units. For example, '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.
1, oneline
Give only one line of output (the forward conversion). Do not print the reverse
conversion. If a reciprocal conversion is performed then 'units' will still print
the ``reciprocal conversion'' line.
t, terse
Give terse output when converting units. This option can be used when calling
'units' from another program so that the output is easy to parse. This option has
the combined effect of these options: 'strict' 'quiet' 'oneline'
'compact'.
v, verbose
Give slightly more verbose output when converting units. When combined with the
'c' option this gives the same effect as 'checkverbose'.
V, version
Print program version number, tell whether the 'readline' library has been
included, and give the location of the default units data file.
l locale, locale locale
Force a specified locale such as 'en_GB' to get British definitions by default.
This overrides the locale determined from system settings or environment variables.
See Locale for a description of locale format.
ADDING YOUR OWN DEFINITIONS
Units Data Files
The units and prefixes that 'units' can convert are defined in the units data file, typi
cally '/usr/share/units/definitions.units'. Although you can extend or modify this data
file if you have appropriate user privileges, it's usually better to put extensions in
separate files so that the definitions will be preserved when you update 'units'.
You can include additional data files in the units database using the '!include' command
in the standard units data file. For example
!include /usr/local/share/units/local.units
might be appropriate for a sitewide supplemental data file. The location of the
'!include' statement in the standard units data file is important; later definitions
replace earlier ones, so any definitions in an included file will override definitions
before the '!include' statement in the standard units data file. With normal invocation,
no warning is given about redefinitions; to ensure that you don't have an unintended
redefinition, run 'units c' after making changes to any units data file.
If you want to add your own units in addition to or in place of standard or sitewide sup
plemental units data files, you can include them in the '.units' file in your home direc
tory. If this file exists it is read after the standard units data file, so that any def
initions in this file will replace definitions of the same units in the standard data file
or in files included from the standard data file. This file will not be read if any units
files are specified on the command line. (Under Windows the personal units file is named
'unitdef.units'.)
The 'units' program first tries to determine your home directory from the 'HOME' environ
ment variable. On systems running Microsoft Windows, if 'HOME' does not exist, 'units'
attempts to find your home directory from 'HOMEDRIVE' and 'HOMEPATH'. Running 'units V'
will display the location and name of your personal units file.
You can specify an arbitrary file as your personal units data file with the 'MYUNITSFILE'
environment variable; if this variable exists, its value is used without searching your
home directory.
Defining New Units and Prefixes
A unit is specified on a single line by giving its name and an equivalence. Comments
start with a '#' character, which can appear anywhere in a line. The backslash character
('\') acts as a continuation character if it appears as the last character on a line, mak
ing it possible to spread definitions out over several lines if desired. A file can be
included by giving the command '!include' followed by the file's name. The '!' must be
the first character on the line. The file will be sought in the same directory as the
parent file unless you give a full path. The name of the file to be included cannot con
tain the comment character '#'.
Unit names must not contain any of the operator characters '+', '', '*', '/', '', '^',
';', '~', the comment character '#', or parentheses. They cannot begin or end with an
underscore ('_'), a comma (',') or a decimal point ('.'). Names cannot begin with a
digit, and if a name ends in a digit other than zero, the digit must be preceded by a
string beginning with an underscore, and afterwards consisting only of digits, decimal
points, or commas. For example, 'foo_2', 'foo_2,1', or 'foo_3.14' would be valid names
but 'foo2' or 'foo_a2' would be invalid. You could define nitrous oxide as
N2O nitrogen 2 + oxygen
but would need to define nitrogen dioxide as
NO_2 nitrogen + oxygen 2
Be careful to define new units in terms of old ones so that a reduction leads to the prim
itive units, which are marked with '!' characters. Dimensionless units are indicated by
using the string '!dimensionless' for the unit definition.
When adding new units, be sure to use the 'c' option to check that the new units reduce
properly. If you create a loop in the units definitions, then 'units' will hang when
invoked with the 'c' option. You will need to use the 'checkverbose' option, which
prints out each unit as it is checked. The program will still hang, but the last unit
printed will be the unit that caused the infinite loop.
If you define any units that contain '+' characters, carefully check them because the 'c'
option will not catch nonconformable sums. Be careful with the '' operator as well.
When used as a binary operator, the '' character can perform addition or multiplication
depending on the options used to invoke 'units'. To ensure consistent behavior use ''
only as a unary negation operator when writing units definitions. To multiply two units
leave a space or use the '*' operator with care, recalling that it has two possible prece
dence values and may require parentheses to ensure consistent behavior. To compute the
difference of 'foo' and 'bar' write 'foo+(bar)' or even 'foo+bar'.
Here is an example of a short data file that defines some basic units:
m ! # The meter is a primitive unit
sec ! # The second is a primitive unit
rad !dimensionless # A dimensionless primitive unit
micro 1e6 # 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
A unit that ends with a '' character is a prefix. If a prefix definition contains any
'/' characters, be sure they are protected by parentheses. If you define 'half 1/2' then
'halfmeter' would be equivalent to '1 / (2 meter)'.
Defining Nonlinear Units
Some unit conversions of interest are nonlinear; for example, temperature conversions
between the Fahrenheit and Celsius scales cannot be done by simply multiplying by conver
sion factors.
When you give a linear unit definition such as 'inch 2.54 cm' you are providing informa
tion that 'units' uses to convert values in inches into primitive units of meters. For
nonlinear units, you give a functional definition that provides the same information.
Nonlinear units are represented using a functional notation. It is best to regard this
notation not as a function call but as a way of adding units to a number, much the same
way that writing a linear unit name after a number adds units to that number. Internally,
nonlinear units are defined by a pair of functions that convert to and from linear units
in the data file, so that an eventual conversion to primitive units is possible.
Here is an example nonlinear unit definition:
tempF(x) units=[1;K] (x+(32)) degF + stdtemp ; \
(tempF+(stdtemp))/degF + 32
A nonlinear unit definition comprises a unit name, a dummy parameter name, two functions,
and two corresponding units. The functions tell 'units' how to convert to and from the
new unit. In order to produce valid results, the arguments of these functions need to
have the correct dimensions. To facilitate error checking, you may optionally indicate
units for these arguments.
The definition begins with the unit name followed immediately (with no spaces) by a '('
character. In parentheses is the name of the parameter. Next is an optional specifica
tion of the units required by the functions in this definition. In the example above, the
'tempF' function requires an input argument conformable with '1'. For normal nonlinear
units definitions the forward function will always take a dimensionless argument. The
inverse function requires an input argument conformable with 'K'. In general the inverse
function will need units that match the quantity measured by your nonlinear unit. The
purpose of the expression in brackets to enable 'units' to perform error checking on func
tion arguments, and also to assign units to range and domain specifications, which are
described later.
Next the function definitions appear. In the example above, the 'tempF' function is
defined by
tempF(x) = (x+(32)) degF + stdtemp
This gives a rule for converting 'x' in the units 'tempF' to linear units of absolute tem
perature, which makes it possible to convert from tempF to other units.
In order to make conversions to Fahrenheit possible, you must give a rule for the inverse
conversions. The inverse will be 'x(tempF)' and its definition appears after a ';' charac
ter. In our example, the inverse is
x(tempF) = (tempF+(stdtemp))/degF + 32
This inverse definition takes an absolute temperature as its argument and converts it to
the Fahrenheit temperature. The inverse can be omitted by leaving out the ';' character,
but then conversions to the unit will be impossible. If the inverse is omitted then the
'check' option will display a warning. It is up to you to calculate and enter the
correct inverse function to obtain proper conversions. The 'check' option tests the
inverse at one point and prints an error if it is not valid there, but this is not a guar
antee that your inverse is correct.
If you wish to make synonyms for nonlinear units, you still need to define both the for
ward and inverse functions. Inverse functions can be obtained using the '~' operator. So
to create a synonym for 'tempF' you could write
fahrenheit(x) units=[1;K] tempF(x); ~tempF(fahrenheit)
You may define a function whose range and domain do not cover all of the real numbers. In
this case 'units' can handle errors better if you specify an appropriate range and domain.
You specify the range and domain as shown below.
baume(d) units=[1;g/cm^3] domain=[0,130.5] range=[1,10] \
(145/(145d)) g/cm^3 ; (baume+g/cm^3) 145 / baume
In this example the domain is specified after the 'domain=' with the endpoints given in
brackets. One of the end points can be omitted to get an interval that goes to infinity.
So the range could be specified as nonnegative by writing 'range=[0,]'. Both the range
and domain are optional and can appear independently and in any order along with the
'units' specification. The values in the range and domain are attached to the units given
in the 'units' specification. If you don't specify the units then the parameter inputs
are reduced to primitive units for the numeric comparison to the values you give in the
range or domain. In this case you should only use 'range' or 'domain' if the endpoints
are zero and infinity.
Specifying the range and domain allows 'units' to perform better error checking and give
more helpful error messages when you invoke nonlinear units conversions outside of their
bounds. It also enables the 'c' option to find a point in the domain to use for its
point check of your inverse definition.
You may occasionally wish to define a function that operates on units. This can be done
using a nonlinear unit definition. For example, the definition below provides conversion
between radius and the area of a circle. This definition requires a length as input and
produces an area as output, as indicated by the 'units=' specification. Specifying the
range as the nonnegative numbers can prevent cryptic error messages.
circlearea(r) units=[m;m^2] range=[0,] pi r^2 ; sqrt(circlearea/pi)
Sometimes you may be interested in a piecewise linear unit such as many wire gauges.
Piecewise linear units can be defined by specifying conversions to linear units on a list
of points. Conversion at other points will be done by linear interpolation. A partial
definition of zinc gauge is
zincgauge[in] 1 0.002, 10 0.02, 15 0.04, 19 0.06, 23 0.1
In this example, 'zincgauge' is the name of the piecewise linear unit. The definition of
such a unit is indicated by the embedded '[' character. After the bracket, you should
indicate the units to be attached to the numbers in the table. No spaces can appear
before the ']' character, so a definition like 'foo[kg meters]' is illegal; instead write
'foo[kg*meters]'. The definition of the unit consists of a list of pairs optionally sepa
rated by commas. This list defines a function for converting from the piecewise linear
unit to linear units. The first item in each pair is the function argument; the second
item is the value of the function at that argument (in the units specified in brackets).
In this example, we define 'zincgauge' at five points. For example, we set 'zincgauge(1)'
equal to '0.002 in'. Definitions like this may be more readable if written using con
tinuation characters as
zincgauge[in] \
1 0.002 \
10 0.02 \
15 0.04 \
19 0.06 \
23 0.1
With the preceding definition, the following conversion can be performed:
You have: zincgauge(10)
You want: in
* 0.02
/ 50
You have: .01 inch
You want: zincgauge
5
If you define a piecewise linear unit that is not strictly monotonic, then the inverse
will not be well defined. If the inverse is requested for such a unit, 'units' will
return the smallest inverse. The 'check' option will print a warning if a nonmonotonic
piecewise linear unit is encountered.
Defining Unit List Aliases
Unit list aliases are treated differently from unit definitions, because they are a data
entry shorthand rather than a true definition for a new unit. A unit list alias defini
tion begins with '!unitlist' and includes the alias and the definition; for example, the
aliases included in the standard units data file are
!unitlist hms hr;min;sec
!unitlist time year;day;hr;min;sec
!unitlist dms deg;arcmin;arcsec
!unitlist ftin ft;in;18 in
!unitlist usvol cup;34 cup;23 cup;12 cup;13 cup;14 cup;\
tbsp;tsp;12 tsp;14 tsp;18 tsp
Unit list aliases are only for unit lists, so the definition must include a ';'. Unit
list aliases can never be combined with units or other unit list aliases, so the defini
tion of 'time' shown above could not have been shortened to 'year;day;hms'. As usual, be
sure to run 'units check' to ensure that the units listed in unit list aliases are con
formable.
NUMERIC OUTPUT FORMAT
By default, results of conversions are shown to eight significant figures; this can be
changed with the 'exponential' and 'outputformat' options. The former sets an expo
nential format (i.e., scientific notation) like that used in the original Unix 'units'
program; the latter allows the format to be given as that of the printf function in the
ANSI C standard.
The format recognized with the 'outputformat' option is a subset of that for printf().
Only a floatingpoint format of the form '%'[flag][width]['.'precision]type is allowed: it
must begin with '%', and must end with a floatingpoint type specifier ('E' or 'e' for
scientific notation, 'f' for fixedpoint decimal, or 'G' or 'g' to specify the number of
significant figures). The format specification may include one optional flag ('+', '',
'#', or a space), followed by an optional value for the minimum field width, and an
optional precision specification that begins with a period (e.g., '.6'). In addition to
the digits, the field width includes the decimal point, the exponent, and the sign if any
of these are shown. A width specification is typically used with fixedpoint decimal to
have columns of numbers align at the decimal point; it normally is not useful with
'units'. Nonfloatingpoint type specifiers make no sense for 'units', and are forbidden.
The default format is '%.8g'; for greater precision, you could specify 'o %.15g'. The
'G' and 'g' formats use exponential format whenever the exponent would be less than 5, so
the value 0.000013 displays as '1.3e005'. If you prefer fixedpoint display, you might
specify 'o %.8f'; however, very small numbers may display very few significant figures,
and for very small numbers, may show nothing but zeros.
See the documentation for printf() for more detailed descriptions of the format specifica
tion.
LOCALIZATION
Some units have different values in different locations. The localization feature accom
modates this by allowing a units data file to specify definitions that depend on the
user's locale.
Locale
A locale is a subset of a user's environment that indicates the user's language and coun
try, and some attendant preferences, such as the formatting of dates. The 'units' program
attempts to determine the locale from the POSIX setlocale function; if this cannot be
done, 'units' examines the environment variables 'LC_CTYPE' and 'LANG'. On POSIX systems,
a locale is of the form language'_'country, where language is the twocharacter code from
ISO 6391 and country is the twocharacter code from ISO 31661; language is lower case
and country is upper case. For example, the POSIX locale for the United Kingdom is
'en_GB'.
On systems running Microsoft Windows, the value returned by setlocale() is different from
that on POSIX systems; 'units' attempts to map the Windows value to a POSIX value by means
of a table in the file 'locale.map' in the same directory, typically
'/usr/local/share/units', as the default units data files. The file includes entries for
many combinations of language and country, and can be extended to include other combina
tions. The 'locale.map' comprises two tabseparated columns; each entry is of the form
Windowslocale POSIXlocale
where POSIXlocale is as described above, and Windowslocale typically spells out both the
language and country. For example, the entry for the United States is
English_United States en_US
You can force 'units' to run in a desired locale by using the 'l' option.
In order to create unit definitions for a particular locale you begin a block of defini
tions in a unit datafile with '!locale' followed by a locale name. The '!' must be the
first character on the line. The 'units' program reads the following definitions only if
the current locale matches. You end the block of localized units with '!endlocale'. Here
is an example, which defines the British gallon.
!locale en_GB
gallon 4.54609 liter
!endlocale
Additional Localization
Sometimes the locale isn't sufficient to determine unit preferences. There could be
regional preferences, or a company could have specific preferences. Though probably
uncommon, such differences could arise with the choice of English customary units outside
of Englishspeaking countries. To address this, 'units' allows specifying definitions
that depend on environment variable settings. The environment variables can be controled
based on the current locale, or the user can set them to force a particular group of defi
nitions.
A conditional block of definitions in a units data file begins with either '!var' or
'!varnot' following by an environment variable name and then a space separated list of
values. The leading '!' must appear in the first column of a units data file, and the
conditional block is terminated by '!endvar'. Definitions in blocks beginning with '!var'
are executed only if the environment variable is exactly equal to one of the listed val
ues. Definitions in blocks beginning with '!varnot' are executed only if the environment
variable does not equal any of the list values.
The inch has long been a customary measure of length in many places. The word comes from
the latin uncia meaning ``one twelfth,'' referring to its relationship with the foot. By
the 20th century, the inch was officially defined in Englishspeaking countries relative
to the yard, but until 1959, the yard differed slightly among those countries. In France
the customary inch, which was displaced in 1799 by the meter, had a different length based
on a french foot. These customary definitions could be accomodated as follows:
!var INCH_UNIT usa
yard 36003937 m
!endvar
!var INCH_UNIT canada
yard 0.9144 meter
!endvar
!var INCH_UNIT uk
yard 0.91439841 meter
!endvar
!var INCH_UNIT canada uk usa
foot 13 yard
inch 112 foot
!endvar
!var INCH_UNIT france
foot 144443.296 m
inch 112 foot
line 112 inch
!endvar
!varnot INCH_UNIT usa uk france canada
!message Unknown value for INCH_UNIT
!endvar
When 'units' reads the above definitions it will check the environment variable
'INCH_UNIT' and load only the definitions for the appropriate section. If 'INCH_UNIT' is
unset or is not set to one of the four values listed then 'units' will run the last block.
In this case that block uses the '!message' command to display a warning message. Alter
natively that block could set default values.
In order to create default values that are overridden by user settings the data file can
use the '!set' command, which sets an environment variable only if it is not already set;
these settings are only for the current 'units' invocation and do not persist. So if the
example above were preceded by '!set INCH_UNIT france' then this would make 'france' the
default value for 'INCH_UNIT'. If the user had set the variable in the environment before
invoking 'units', then 'units' would use the user's value.
To link these settings to the user's locale you combine the '!set' command with the
'!locale' command. If you wanted to combine the above example with suitable locales you
could do by preceding the above definition with the following:
!locale en_US
!set INCH_UNIT usa
!endlocale
!locale en_GB
!set INCH_UNIT uk
!endlocale
!locale en_CA
!set INCH_UNIT canada
!endlocale
!locale fr_FR
!set INCH_UNIT france
!endlocale
!set INCH_UNIT france
These definitions set the overall default for 'INCH_UNIT' to 'france' and set default val
ues for four locales appropriately. The overall default setting comes last so that it
only applies when 'INCH_UNIT' was not set by one of the other commands or by the user.
If the variable given after '!var' or '!varnot' is undefined then 'units' prints an error
message and ignores the definitions that follow. Use '!set' to create defaults to prevent
this situation from arising. The 'c' option only checks the definitions that are active
for the current environment and locale, so when adding new definitions take care to check
that all cases give rise to a well defined set of definitions.
ENVIRONMENT VARIABLES
The 'units' program uses the following environment variables:
HOME Specifies the location of your home directory; it is used by 'units' to find a per
sonal units data file '.units'. On systems running Microsoft Windows, 'units'
tries to determine your home directory from the 'HOMEDRIVE' and 'HOMEPATH' environ
ment variables if 'HOME' does not exist.
LC_CTYPE, LANG
Checked to determine the locale if 'units' cannot obtain it from the operating sys
tem. Sections of the standard units data file are specific to certain locales.
MYUNITSFILE
Specifies your personal units data file. If this variable exists, 'units' uses its
value rather than searching your home directory for '.units'. The personal units
file will not be loaded if any data files are given using the 'f' option.
PAGER Specifies the pager to use for help and for displaying the conformable units. The
help function browses the units database and calls the pager using the '+n'n syntax
for specifying a line number. The default pager is 'more'; 'PAGER' can be used to
specify alternatives such as 'less', 'pg', 'emacs', or 'vi'.
UNITS_ENGLISH
Set to either 'US' or 'GB' to choose United States or British volume definitions,
overriding the default from your locale.
UNITSFILE
Specifies the units data file to use (instead of the default). You can only spec
ify a single units data file using this environment variable. If units data files
are given using the 'f' option, the file specified by 'UNITSFILE' will be not be
loaded unless the 'f' option is given with the empty string ('units f ""').
UNICODE SUPPORT
The standard units data file is written in Unicode using the UTF8 encoding. Portions of
the file that are not plain ASCII begin with '!utf8' and end with '!endutf8'. As usual,
the '!' must appear as the first character on the line. If a line of a data file con
tains byte sequences that are invalid UTF8 or nonprinting UTF8 then 'units' ignores the
entire line.
When 'units' runs it checks the locale to determine the character set. If UTF8 is
listed, then 'units' reads the utf8 definitions. If any other character set is in use,
then 'units' works in plain ASCII without support for extended characters.
READLINE SUPPORT
If the 'readline' package has been compiled in, then when 'units' is used interactively,
numerous command line editing features are available. To check if your version of 'units'
includes 'readline', invoke the program with the 'version' option.
For complete information about 'readline', consult the documentation for the 'readline'
package. Without any configuration, 'units' will allow editing in the style of emacs. Of
particular use with 'units' are the completion commands.
If you type a few characters and then hit ESC followed by '?' then 'units' will display a
list of all the units that start with the characters typed. For example, if you type
'metr' and then request completion, you will see something like this:
You have: metr
metre metriccup metrichorsepower metrictenth
metretes metricfifth metricounce metricton
metriccarat metricgrain metricquart metricyarncount
You have: metr
If there is a unique way to complete a unitname, you can hit the TAB key and 'units' will
provide the rest of the unit name. If 'units' beeps, it means that there is no unique
completion. Pressing the TAB key a second time will print the list of all completions.
UPDATING CURRENCY EXCHANGE RATES
The units program includes currency exchange rates and prices for some precious metals in
the database. Of course, these values change over time, sometimes very rapidly, and
'units' cannot provide real time values. To update the exchange rates run the
'units_cur', which rewrites the files containing the currency rates, typically
'/usr/local/share/units/currency.units'. This program must be run with suitable permis
sions to write the file. To keep the rates updated automatically, it could be run by a
cron job on a Unixlike system, or a similar scheduling program on a different system.
Currency exchange rates are taken from Time Genie (http://www.timegenie.com) and precious
metals pricing from Packetizer (www.packetizer.com). These sites update once per day, so
there is no benefit in running the update script more often than daily. You can run
'units_cur' with a filename specified on the command line and it will write the data to
that file. If you give '' for the file it will write to standard output.
DATABASE COMMAND SYNTAX
unit definition
Define a regular unit.
prefix definition
Define a prefix.
funcname(var) units=[inunits,outunits] domain=[x1,x2] range=[y1,y2] definition(var) ;
inverse(funcname)
Define a nonlinear unit or unit function. The three optional keywords 'units=',
'range=' and 'domain=' can appear in any order. The definition of the inverse is
optional.
tabname[outunits] pairlist
Define a piecewise linear unit. The pair list gives the points on the table listed
in ascending order.
!endlocale
End a block of definitions beginning with '!locale'
!endutf8
End a block of definitions begun with '!utf8'
!endvar
End a block of definitions begun with '!var' or '!varnot'
!include file
Include the specified file.
!locale value
Load the following definitions only of the locale is set to value.
!message text
Display text when the database is read unless the quiet option ('q') is enabled.
!set variable value
Sets the environment variable, variable, to the specified value only if it is not
already set.
!unitlist alias definition
Define a unit list alias.
!utf8 Load the following definitions only if 'units' is running with UTF8 enabled.
!var variable valuelist
Load the following definitions only if the environment variable, variable is set to
one of the values listed on the space separated value list. If variable is not set
then 'units' prints an error message and ignores the following definitions.
!varnot variable valuelist
Load the following definitions only if the environment variable, variable is not
set to one of the values listed on the space separated value list. If variable is
not set then 'units' prints an error message and ignores the following definitions.
GNU FREE DOCUMENTATION LICENSE
FILES
/usr/share/units/definitions.units  the standard units data file
AUTHO
27 April 2012 UNITS(1) 
