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 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.