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Manual Pages  — UNITS


units – conversion program



units [-f filename] [-H filename] [-qvUV] [from-unit to-unit]


The following options are available:
-h -No -, --help
  Show an overview of options
-f filename No , --file filename
  Specify the name of the units data file to load.
-H filename No , --history filename
  Ignored, for compatibility with GNU units.
-e -, --exponential
  Behave as if -o '%6e' was typed.
-q -No -, --quiet
  Suppress prompting of the user for units and the display of statistics about the number of units loaded.
-U -No -, --unitsfile
  If the default unit file exists prints its location. If not, print " Units data file not found "
-t -No -, --terse
  Only print the result. This is used when calling units from other programs for easy to parse results.
-v -No -, --verbose
  Print the units in the conversion output. Be more verbose in general.
-o format No , --output-format format
  Select the output format string by which numbers are printed.
-V -No -, --version
  Print the version number, usage, and then exit.
from-unit to-unit
  Allow a single unit conversion to be done directly from the command line. The program will not print prompts. It will print out the result of the single specified conversion.


The units program converts quantities expressed in various scales to their equivalents in other scales. The units program can only handle multiplicative or affine scale changes. It 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

You have: meters/s You want: furlongs/fortnight * 6012.8848 / 0.00016630952

You have: 1|2 inch You want: cm * 1.27 / 0.78740157

You have: 85 degF You want: degC         29.444444

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'. Division of numbers must be indicated using the vertical bar ('|'). To convert half a meter, you would write '1|2 meter'. If you write '1/2 meter' then the units program would interpret that as equivalent to '0.5/meter'. 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 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:

ratio of circumference to diameter

speed of light

charge on an electron

acceleration of gravity

same as g

Avogadro's number

pressure per unit height of water

pressure per unit height of mercury

astronomical unit

The unit 'pound' is a unit of mass. Compound names are run together so 'pound force' is a unit of force. The unit 'ounce' is also a unit of mass. The fluid ounce is 'floz'. 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 units will try to remove a trailing 's' or a trailing 'es' and check again for a match.

To find out what units are available read the standard units 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. 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. Comments in the unit definition file begin with a '#' or '/' character at the beginning of a line.

Prefixes are defined in the same was as standard units, but with a trailing dash at the end of the prefix name. If a unit is not found even after removing trailing 's' or 'es', then it will be checked against the list of prefixes. Prefixes will be removed until a legal base unit is identified.

Here is an example of a short units file that defines some basic units.




60 sec

60 min

0.0254 m

12 inches

5280 ft


  the standard units library


Adrian Mariano <Mt adrian@cam.cornell.edu>


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 does not take so long to load the units list and check for duplication.

UNITS (1) August 12, 2017

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