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NAME

units — conversion program

SYNOPSIS

DESCRIPTION

The units program converts quantities expressed in various scales to their equivalents in other scales. The units program can only handle multiplicative scale changes. It cannot convert Celsius to Fahrenheit, for example. It also does not handle logarithmic units such as bels.

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

That is, one meter equals roughly 3.28 feet (or, conversely, one foot is roughly equal to 0.3 meters), and one gallon is roughly equal to 3785 cubic cm.

The units program can handle numbers as well:

You have: 60 miles/hr
You want: km/hr
        * 96.56064
        / 0.010356187

You have: 100 USD
You want: 10 euro
        * 7.4354971
        / 0.13449

In other words, 60 miles per hour is about 96.6 km/hr, and 100 US dollars will get you seven 10-Euro notes.

The options are as follows:

-f filename

Specifies the name of the units data file to load. The standard units library is read if filename is the empty string. This option may be specified multiple times to extend the standard units library with a personal library.

-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. The units program will print out only the result of this single conversion. A count can be given to specify multiples of from-unit.

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:

‘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. The units program 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. Prefixes are applied after the longest matching unit name is found; for example, “nmile” is taken to be a nautical mile rather than a nanomile.

FILES

/usr/share/misc/units.lib

the standard units library

SEE ALSO

The International System of Units, International Bureau of Weights and Measures, https://www.bipm.org/utils/common/pdf/si_brochure_8.pdf.

Codes for the representation of currencies and funds, ISO 4217, 2008.

Quantities and units — Part 13: Information science and technology, ISO/IEC 80000.

AUTHORS

Adrian Mariano <adrian@cam.cornell.edu> or <mariano@geom.umn.edu>

BUGS

The effect of including a ‘/’ in a prefix is surprising.

Exponents of units 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.

Prefixes specified without a unit are treated as dimensionless quantities. This can lead to confusion when some prefixes are also defined as units (e.g., m). For example, Tera- / Giga- is 1000, but one Tesla (T) is 10,000 Gauss (G).

Some non-SI units have multiple definitions (e.g., barrel, calorie) and others have changed over time (e.g., cubit). In particular, monetary values fluctuate.

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.