NAME
units - unit conversion program
OVERVIEW OF ‘UNITS’
The ‘units’ program converts quantities expressed in various scales to
their equivalents in other scales. The ‘units’ program can handle
multiplicative scale changes as well as nonlinear conversions such as
Fahrenheit to Celsius. Temperature conversions require a special
syntax. 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.
INTERACTING WITH ‘UNITS’
To invoke units for interactive use, type ‘units’ at your shell prompt.
The program will print something like this:
2131 units, 53 prefixes, 24 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 type of units you want to convert to. To convert to
feet, you would type ‘feet’. Note that 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.
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 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 ‘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:
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 which measure reciprocal
dimensions, then ‘units’ will display the conversion results with an
extra note indicating that reciprocal conversion 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
2.7777778e-11 kg m^2 / sec^3
2.1166667e-05 kg^2 m / sec
If you only want to find the reduced form or definition of a unit,
simply press return at the ‘You want:’ prompt. Here is an example:
You have: jansky
You want:
Definition: fluxunit = 1e-26 W/m^2 Hz = 1e-26 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 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
Note that named dimensionaless 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 which are
conformable with the unit that you entered at the ‘You have:’ prompt
above. Note that 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.
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’ NON-INTERACTIVELY
The ‘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
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 (0)
to the calling environment. If ‘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 ‘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.
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 ‘^’ character as shown in the following
example, or by simple concatenation: ‘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. The ‘^’ operator has the
second highest precedence. The ‘**’ operator is provided as an
alternative 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
Multiplication of units can be specified by using spaces, or an
asterisk (‘*’). If ‘units’ is invoked with the ‘--product’ option then
the hyphen (‘-’) also acts as a multiplication operator. Division of
units is indicated by the slash (‘/’) or by ‘per’.
You have: furlongs per fortnight
You want: m/s
* 0.00016630986
/ 6012.8727
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 (‘*’) now has the same
precedence as division and hence follows the usual precedence rules.
If units is invoked with the the ‘--oldstar’ option then then the old
behavior is activated and ‘*’ 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 ‘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 .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 (‘|’), 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 ‘2|3^1|2’.
You have: 1|2 inch
You want: cm
* 1.27
/ 0.78740157
Parentheses can be used for grouping as desired.
You have: (1/2) kg / (kg/meter)
You want: league
* 0.00010356166
/ 9656.0833
Prefixes are defined separately from base units. In order to get
centimeters, the units database defines ‘centi-’ and ‘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
‘cm^3’ or ‘centimeter^3’ refer to cubic centimeters but ‘centi*meter^3’
refers to hundredths of cubic meters. Only one prefix is permitted per
unit, so ‘micromicrofarad’ will fail, but ‘micro*microfarad’ will work,
as will ‘micro microfarad’..
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 which 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 dollars^5.
Outside of the SI system, it is often desirable to add values of
different units together. You may also wish to use ‘units’ as a
calculator that keeps track of units. Sums of conformable units are
written 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 which are added together must reduce to identical
expressions in primitive units, or an error message will be displayed:
You have: 12 printerspoint + 4 heredium
^
Illegal sum of non-conformable units
Historically ‘-’ has been used for products of units, which complicates
its iterpretation in ‘units’. 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. This behavior can be altered using
the ‘--product’ option which causes ‘units’ to treat the binary ‘-’
operator as a product operator. Note that when ‘-’ is a multiplication
operator it has the same precedence as ‘*’, but when ‘-’ is a
subtraction operator it has the lower precedence as the addition
operator.
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.
The ‘+’ character 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 ‘+’ as part of an exponent if possible.
Several built in functions are provided: ‘sin’, ‘cos’, ‘tan’, ‘ln’,
‘log’, ‘log2’, ‘exp’, ‘acos’, ‘atan’ and ‘asin’. The ‘sin’, ‘cos’, and
‘tan’ functions require either a dimensionless 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 trigonometric 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. Higher roots can be obtained 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
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 ‘temp’, and you must use functional
notation. The temperature differences are done using units starting
with ‘deg’ and they do not require functional 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 which indicates
that ‘x’ should have units of ‘tempF’ attached to it. See 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 ‘g00’, ‘g000’, and so on that
are defined in the units database.
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
INVOKING ‘UNITS’
You invoke ‘units’ like this:
units [OPTIONS] [FROM-UNIT [TO-UNIT]]
If the FROM-UNIT and TO-UNIT are omitted, then the program will use
interactive prompts to determine which conversions to perform. See
Interactive use. If both FROM-UNIT and TO-UNIT are given, ‘units’ will
print the result of that single conversion and then exit. If only
FROM-UNIT appears on the command line, ‘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. 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 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.
--check-verbose
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. Note that only definitions active in the current
locale are checked.
-o format, --output-format format
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 ‘-o %.15g’.
-f filename, --file filename
Instruct ‘units’ to load the units file ‘filename’. If
‘filename’ is the empty string (‘-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
‘UNITSFILE’ environment variable.
-h, --help
Print out a summary of the options for ‘units’.
-m, --minus
Causes ‘-’ to be interpreted as a subtraction operator. This is
usually the default behavior.
-p, --product
Causes ‘-’ to be interpreted as a multiplication operator when
it has two operands. It will as a negation operator when it has
only one operand: ‘(-3)’. Note that by default ‘-’ is treated
as a subtraction operator.
, --oldstar Causes ‘*’ to have the old style precedence, higher than
the precedence of division so that ‘1/2*3’ will equal ‘6’.
, --newstar Forces ‘*’ to have the new (default) precedence which
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.
-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, --one-line
Give only one line of output (the forward conversion). Do not
print the reverse conversion. Note that if a reciprocal
conversion is performed then ‘units’ will print 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’ ‘--one-line’ ‘--compact’.
-v, --verbose
Give slightly more verbose output when converting units. When
combined with the ‘-c’ option this gives the same effect as
‘--check-verbose’.
-V, --version
Print program version number, tell whether the readline library
has been included, and give the location of the default units
data file.
UNIT DEFINITIONS
The conversion information is read from a units data file which is
called ‘units.dat’ and is probably located in the ‘/usr/local/share’
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 permitivity of vacuum
G gravitational constant
mach speed of sound
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 ‘2 cups flour_sifted’ can be
converted to ‘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 ‘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’.
The US Survey foot, yard, and mile can be obtained by using the ‘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 ‘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 ‘UK’.
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’ or a trailing ‘es’. If that fails, ‘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.
DEFINING NEW UNITS
All of the units and prefixes that ‘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
‘.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 ‘#’ 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,
making 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 file will be sought in the same
directory as the parent file unless a full path is given.
Unit names must not contain any of the operator characters ‘+’, ‘-’,
‘*’, ‘/’, ‘|’, ‘^’ or the parentheses. They cannot begin with a digit
or a decimal point (‘.’), 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 ‘!’
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’ options.
You will need to use the ‘--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 ‘+’ characters, carefully check
them because the ‘-c’ option will not catch non-conformable 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 precedence 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 units file that defines some basic units:
m ! # The meter is a primitive unit
sec ! # The second is a primitive unit
rad !dimensionless # The second is a 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
A unit which 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 units conversions of interest are nonlinear; for example,
temperature conversions between the Fahrenheit and Celsius scales
cannot be done by simply multiplying by conversions factors.
When you give a linear unit definition such as ‘inch 2.54 cm’ you are
providing information 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 which 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) [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 specify the
dimensions.
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 specification 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 sole purpose of the expression in brackets to enable ‘units’ to
perform error checking on function arguments.
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 temperature, 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 ‘;’ character. 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 print an error if
it is not valid there, but this is not a guarantee that your inverse is
correct.
If you wish to make synonyms for nonlinear units, you still need to
define both the forward and inverse functions. Inverse functions can
be obtained using the ‘~’ operator. So to create a synonym for ‘tempF’
you could write
fahrenheit(x) [1;K] tempF(x); ~tempF(fahrenheit)
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. Note that this definition requires a length as input and
produces an area as output, as indicated by the specification in
brackets.
circlearea(r) [m;m^2] 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 separated 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 continuation
characters as
zincgauge[in] \
1 0.002 \
10 0.02 \
15 0.04 \
19 0.06 \
23 0.1
With the preceeding 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 non-monotonic piecewise
linear unit is encountered.
LOCALIZATION
Some units have different values in different locations. The
localization feature accomodates this by allowing the units database to
specify region dependent definitions. A locale region in the units
database begins with ‘!locale’ followed by the name of the locale. The
leading ‘!’ must appear in the first column of the units database. The
locale region is terminated by ‘!endlocale’. The following example
shows how to define a couple units in a locale.
!locale en_GB
ton brton
gallon brgallon
!endlocale
The current locale is specified by the ‘LOCALE’ environment variable.
Note that the ‘-c’ option only checks the definitions which are active
for the current locale.
ENVIRONMENT VARIABLES
The ‘units’ programs uses the following environment variables.
LOCALE Specifies the locale. The default is ‘en_US’. Sections of the
units database are specific to certain locales.
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 ‘+nn’ syntax for specifying a line
number. The default pager is ‘more’, but ‘less’, ‘emacs’, or
‘vi’ are possible alternatives.
UNITSFILE
Specifies the units database file to use (instead of the
default). This will be overridden by the ‘-f’ option. Note that
you can only specify a single units database using this
environment variable.
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 the 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 the ‘?’ key
then ‘units’ will display a list of all the units which 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.
FILES
/usr/share/misc/units.dat - the standard units data file
AUTHOR
Adrian Mariano (adrian@cam.cornell.edu)
25 Sep 2007