The units and prefixes that units can convert are defined in
the units data file, typically /usr/share/units/definitions.units.
If you can’t find this file, run units --version to get
information on the file locations for your installation.
Although you can extend or modify this data file if you have appropriate
user privileges, it’s usually better to put extensions in separate files
so that the definitions will be preserved if you update units.
You can include additional data files in the units database using the ‘!include’ command in the standard units data file. For example
!include /usr/local/share/units/local.units
might be appropriate for a site-wide supplemental data file.
The location of the ‘!include’ statement in the standard units
data file is important; later definitions replace earlier ones,
so any definitions in an included file will override definitions before
the ‘!include’ statement in the standard units data file.
With normal invocation, no warning is given about redefinitions; to
ensure that you don’t have an unintended redefinition, run
units -c after making changes to any units data file.
If you want to add your own units in addition to or in place of
standard or site-wide supplemental units data files, you can include
them in the .units file in your home directory. If this
file exists it is read after the standard units data file, so that any
definitions in this file will replace definitions of the same units in
the standard data file or in files included from the standard data
file. This file will not be read if any units files are specified on
the command line. (Under Windows the personal units file is
named unitdef.units.) Running units -V will
display the location and name of your personal units file.
The units program first tries to determine your home
directory from the HOME environment variable. On systems running
Microsoft Windows, if HOME does not exist, units
attempts to find your home directory from HOMEDRIVE,
HOMEPATH and USERPROFILE.
You can specify an arbitrary file as your personal units data file with
the MYUNITSFILE environment variable; if this variable exists, its
value is used without searching your home directory.
The default units data files are described in more detail in
Data Files.