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(units.info)Defining new units

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Defining new units
******************

   All of the units and prefixes that `units' can convert are defined
in the units data 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.  Comments start with a `#' character, which can appear
anywhere in a line.  The backslash character (`\') acts as a
continuation character if it appears as the last character on a line,
making it possible to spread definitions out over several lines if
desired.

   Unit names must not contain any of the operator characters `+', `-',
`*', `/', `|', `^' or the parentheses.  They cannot begin with a digit
or a decimal point (`.'), nor can they end with a digit (except for
zero).  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.  When adding new units, be sure to use the `-c' option to
check that the new units reduce properly.  If you define any units
which contain `+' characters, carefully check them because the `-c'
option will not catch non-conformable sums.  If you create a loop in
the units definitions, then `units' will hang when invoked with the
`-c' options.  You will need to use the `--check-verbose' option which
prints out each unit as it checks them.  The program will still hang,
but the last unit printed will be the unit which caused the infinite
loop.

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

     m       !               # The meter is a primitive unit
     sec     !               # The second is a primitive unit
     micro-  1e-6            # Define a prefix
     minute  60 sec          # A minute is 60 seconds
     hour    60 min          # An hour is 60 minutes
     inch    0.0254 m        # Inch defined in terms of meters
     ft      12 inches       # The foot defined in terms of inches
     mile    5280 ft         # And the mile

A unit which ends with a `-' character is a prefix.  If a prefix
definition contains any `/' characters, be sure they are protected by
parentheses.  If you define `half- 1/2' then `halfmeter' would be
equivalent to `1 / 2 meter'.