.\"	$NetBSD: units.1,v 1.17 2011/04/03 19:44:15 dholland Exp $
.Dd April 3, 2011
.Dt UNITS 1
.Os
.Sh NAME
.Nm units
.Nd conversion program
.Sh SYNOPSIS
.Nm
.Op Fl f Ar filename
.Op Fl qv
.Oo
.Op Ar count
.Ar from-unit to-unit
.Oc
.Sh DESCRIPTION
.Nm
converts quantities expression in various scales to
their equivalents in other scales.
.Nm
can only handle multiplicative scale changes.
It cannot convert Centigrade to Fahrenheit, for example.
.Pp
The following options are supported:
.Bl -tag -width "from-unit to-unit" -offset indent
.It Fl f Ar filename
Specifies the name of the units data file to load.
.It Fl q
Suppresses prompting of the user for units and the display of statistics
about the number of units loaded.
.It Fl v
Prints the version number.
.It Ar from-unit Ar to-unit
Allows a single unit conversion to be done directly from the command
line.
No prompting will occur.
.Nm
will print out only the result of this single conversion.
The
.Ar count
argument can be prepended to the
.Ar from-unit
or it can be separate.
.El
.Pp
.Nm
works interactively by prompting the user for input:
.Bd -literal
    You have: meters
    You want: feet
            * 3.2808399
            / 0.3048

    You have: cm^3
    You want: gallons
            * 0.00026417205
            / 3785.4118
.Ed
.Pp
Powers of units can be specified using the
.Dq \&^
character as shown in the example, or by simple concatenation:
.Dq cm3
is equivalent to
.Dq cm^3 .
Multiplication of units can be specified by using spaces, a dash or
an asterisk.
Division of units is indicated by the slash
.Pq Sq \&/ .
Note that multiplication has a higher precedence than division,
so
.Dq m/s/s
is the same as
.Dq m/s^2
or
.Dq "m/s s" .
If the user enters incompatible unit types, the
.Nm
program will print a message indicating that the units are not
conformable and it will display the reduced form for each unit:
.Bd -literal
    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
.Ed
.Pp
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:
.Bl -tag -width mercury -compact -offset indent
.It pi
ratio of circumference to diameter
.It c
speed of light
.It e
charge on an electron
.It g
acceleration of gravity
.It force
same as g
.It mole
Avogadro's number
.It water
pressure per unit height of water
.It mercury
pressure per unit height of mercury
.It au
astronomical unit
.El
.Pp
.Dq pound
is a unit of mass.
Compound names are run together
so
.Dq poundforce
is a unit of force.
British units that differ from their
US counterparts are prefixed with
.Dq br ,
and currency is prefixed with
its country name:
.Dq belgiumfranc ,
.Dq britainpound .
When searching for
a unit, if the specified string does not appear exactly as a unit
name, then the
.Nm
program will try to remove a trailing
.Dq s
or a trailing
.Dq es
and check again for a match.
.Pp
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
.Sq \&!
characters.
.Nm
will not detect infinite loops that could be caused
by careless unit definitions.
.Pp
Prefixes are defined in the same way as standard units, but with
a trailing dash at the end of the prefix name.
.Sh FILES
.Bl -tag -width /usr/share/misc/units.lib -compact
.It Pa /usr/share/misc/units.lib
the standard units library
.El
.Sh AUTHORS
.An Adrian Mariano Aq adrian@cam.cornell.edu
or
.Aq mariano@geom.umn.edu
.Sh CAVEATS
While
.Nm
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,
.Nm
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.
.Pp
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
.Nm
to warn about these issues either.
.Sh BUGS
The effect of including a
.Sq \&/
in a prefix is surprising.
.Pp
Exponents entered by the user can be only one digit.
You can work around this by multiplying several terms.
.Pp
The user must use
.Sq \&|
to indicate division of numbers and
.Sq \&/
to indicate division of symbols.
This distinction should not be necessary.
.Pp
The program contains various arbitrary limits on the length
of the units converted and on the length of the data file.
.Pp
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.