units - unit conversion program
OVERVIEW OF `UNITS'
The `units' program converts quantities expressed in vari
ous scales to their equivalents in other scales. The
`units' program can only handle multiplicative scale
changes. For example, it cannot convert Celsius to
Fahrenheit but it can convert temperature differences
between those temperature scales.
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:
1161 units, 53 prefixes
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'.
The answer will be displayed in two ways. The first line
of output, which is marked with a `*' to indicate multi
plication, 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 reverse
directions. 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
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 con
version results with an extra note indicating that recip
rocal 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 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.
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 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 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.
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.
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,
a hyphen (`-') or an asterisk (`*'). Division of units is
indicated by the slash (`/').
You have: furlongs/fortnight
You want: m/s
* 0.00016630986
/ 6012.8727
Multiplication has a higher precedence than division and
is evaluated left to right, so `m/s * s/day' is equivalent
to `m / s s day' and has dimensions of length per time
cubed. In effect, the first `/' character marks the
beginning of the denominator of your unit. In particular,
this means that writing `1/2 meter' refers to a unit of
reciprocal length equivalent to .5/meter, which is proba
bly not what you would intend if you entered that expres
sion. To indicate division of numbers, use the vertical
dash (`|'). No spaces area permitted on either side of
the vertical dash character.
You have: 1|2 inch
You want: cm
* 1.27
/ 0.78740157
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.
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. 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 mes
sage will be displayed:
You have: 12 printerspoint + 4 heredium
Illegal sum of non-conformable units:
12 printerspoint reduces to 0.0042175176 m
4 heredium reduces to 20145.828 m^2
Because `-' is used for products, it cannot also be used
to form differences of units. If a `-' appears before
numerical digits as the very first character on the input
line or if it appears immediately after a `+' then the
number will be evaluated as a negative number. So you can
degrees+-12 arcmin'. The `+' character is sometimes used
in exponents like `3.43e+8'. Exponents of this form can
not be used when forming sums of units, but they may be
used otherwise.
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 conver
sions to perform. If both FROM-UNIT and TO-UNIT are
given, `units' will print the result of that single con
version 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 interpreta
tion and to group them into two arguments. @xref{Command
line use}.
The following options allow you to read in an alternative
units file, check your units file, or change the output
format:
-c, --check
Check that all units and prefixes defined in the
units file reduce to primitive units. The program
will print a list of all units that cannot be
reduced.
--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.
-o format, --output-format format
Use the specified format for numeric output. For
mat 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
Use filename as the units data file rather than the
default units data file.
Print out a summary of the options for `units'.
-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.
-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 ele
ments and numerous other constants. Also included are the
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, multi
ply by the force conversion unit `force' or use the short
hand `lbf'. (Note that `g' is already taken as the stan
dard 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 use until
1959, but for geographic surveys, they are still used.
The acre is officially defined in terms of the US Survey
foot. If you want an acre defined according to the inter
national 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' pro
gram 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.
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. Be careful to define
new units in terms of old ones so that a reduction leads
to the primitive units, which are marked with `!' charac
ters. When adding new units, be sure to use the `-c'
option to check that the new units reduce properly. If
you define any units which contain `+' characters, care
fully check them because the `-c' option will not catch
non-conformable sums. 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'
program will still hang, but the last unit printed will be
the unit which caused the infinite loop.
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
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 contains a `/' character, then when it is applied
to a unit, that unit will be in the denominator. So the
definition `half- 1/2' would give a `halfmeter' equal to
`1 / 2 meter'. For this reason, the `/' character should
be avoided in prefixes.
READLINE SUPPORT
If the `readline' package has been compiled in, then when
`units' is used interactively, numerous command line edit
ing 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 docu
mentation for the readline package. Without any configu
ration, `units' will allow editing in the style of emacs.
Of particular use with `units' are the completion com
mands.
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
FILES
/usr/share/units.dat - the standard units data file
AUTHOR
Adrian Mariano (adrian@cam.cornell.edu)
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