NAME
units
—
conversion program
SYNOPSIS
units |
[-Llqv ] [-f
filename] [[count]
from-unit to-unit] |
DESCRIPTION
units
converts quantities expression in various scales
to their equivalents in other scales. units
can only
handle multiplicative scale changes. It cannot convert Centigrade to
Fahrenheit, for example.
The following options and arguments are supported:
-f
filename- Specifies the name of the units data file to load.
-l
or-L
- List all unit definitions to the standard output, instead of performing
any conversions. The result may include error messages and comments,
beginning with ‘
/
’.With the
-l
option, unit definitions will be listed in a format almost identical to the units data file that was loaded, except that comments will be removed, spacing may be changed, and lines may be re-ordered.With the
-L
option, all unit definitions will be reduced to a form that depends on only a few primitive units (such as m, kg, sec). -q
- Suppresses prompting of the user for units and the display of statistics about the number of units loaded.
-v
- Prints the version number.
- [count] from-unit to-unit
- Allows a single unit conversion to be done directly from the command line.
No prompting will occur.
units
will print out only the result of this single conversion. Specifying count and from-unit as two separate arguments is equivalent to embedding both parts inside a single from-unit argument, with the parts separated by a space.
units
works interactively by prompting the
user for input:
You have: meters You want: feet * 3.2808399 / 0.3048 You have: cm^3 You want: gallons * 0.00026417205 / 3785.4118
Powers of units can be specified using the “^”
character as shown in the example, or by simple concatenation:
“cm3” is equivalent to “cm^3”. Multiplication of
units can be specified by using spaces, a dash or an asterisk. Division of
units is indicated by the slash (‘/’). Note that
multiplication has a higher precedence than division, so
“m/s/s” is the same as “m/s^2” or “m/s
s”. If the user enters 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
The conversion information is read from a units data file. The default file includes definitions for most familiar units, abbreviations and metric prefixes. Some constants of nature included are:
- pi
- ratio of circumference to diameter
- c
- speed of light
- e
- charge on an electron
- g
- acceleration of gravity
- force
- same as g
- mole
- Avogadro's number
- water
- pressure per unit height of water
- mercury
- pressure per unit height of mercury
- au
- astronomical unit
“pound” is a unit of mass. Compound names are run
together so “poundforce” is a unit of force. British units
that differ from their US counterparts are prefixed with “br”,
and 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” or a trailing “es” and check again for a
match.
All of these definitions can be read in the standard units file,
or you can supply your own file. A unit is specified on a single line by
giving its name and an equivalence. One should 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.
units
will not detect infinite loops that could be
caused by careless unit definitions.
Prefixes are defined in the same way as standard units, but with a trailing dash at the end of the prefix name.
FILES
- /usr/share/misc/units.lib
- the standard units library
AUTHORS
Adrian Mariano <adrian@cam.cornell.edu> or ⟨mariano@geom.umn.edu⟩
CAVEATS
While units
can be used as a calculator
for many unit-related computations, caution is required: many computations
require additional constant factors deriving from the physics (or chemistry
or whatever) of the situation. As these factors are dimensionless,
units
cannot itself either provide them or warn the
user when they have been forgotten. For example, one joule is one kilogram
meter squared per second squared, by definition; however, the kinetic energy
of a one-kilogram object moving at one meter per second is half a joule, not
one joule, because of a dimensionless factor that arises from
integration.
Also, some pairs of units that have the same dimensionality are
nonetheless used to measure different things and attempting to convert
between them may require additional fudge factors or be entirely
meaningless. For example, torque and energy have the same dimensionality,
but attempting to convert torque in newton-meters to energy in joules is
nonsensical. There is no practical way for units
to
warn about these issues either.
BUGS
The effect of including a ‘/’ in a prefix is surprising.
Exponents entered by the user can be only one digit. You can work around this by multiplying several terms.
The user must use ‘|’ to indicate division of numbers and ‘/’ to indicate division of symbols. This distinction should not be necessary.
The program contains various arbitrary limits on the length of the units converted and on the length of the data file.
The program should use a hash table to store units so that it doesn't take so long to load the units list and check for duplication.