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# units

## man page of units

### units: unit conversion program

```NAME
units - unit conversion program

OVERVIEW OF 'UNITS'
The  'units' program converts quantities expressed in various scales to
their equivalents in other scales.   The  'units'  program  can  handle
multiplicative  scale  changes as well as nonlinear conversions such as
Fahrenheit to  Celsius.   Temperature  conversions  require  a  special
syntax.  See the examples below.

The  units  are  defined  in  an  external  data file.  You can use the
extensive data file that comes with this program, or  you  can  provide
your own data file to suit your needs.

You  can  use the program interactively with prompts, or you can use it
from the command line.

INTERACTING WITH 'UNITS'
To invoke units for interactive use, type 'units' at your shell prompt.
The program will print something like this:

2131 units, 53 prefixes, 24 nonlinear units

You have:

At  the  'You  have:'  prompt, type the quantity and units that you are
converting from.  For example, if you want to  convert  ten  meters  to
feet,  type  '10  meters'.   Next, 'units' will print 'You want:'.  You
should type the type of units you want to convert to.   To  convert  to
feet,  you  would  type  'feet'.  Note that if the readline library was
compiled in then the tab key can be used to complete unit  names.   See

The  answer  will  be displayed in two ways.  The first line of output,
which is marked with a '*' to indicate multiplication, gives the result
of the conversion you have asked for.  The second line of output, which
is marked with a '/' to indicate division, gives  the  inverse  of  the
conversion  factor.   If  you  convert  10 meters to feet, 'units' will
print

* 32.808399
/ 0.03048

which tells you that 10 meters equals  about  32.8  feet.   The  second
number  gives  the conversion in the opposite direction.  In this case,
it tells you that 1 foot is equal to about 0.03  dekameters  since  the
dekameter is 10 meters.  It also tells you that 1/32.8 is about .03.

The  'units'  program prints the inverse because sometimes it is a more
convenient number.  In the example  above,  for  example,  the  inverse
value is an exact conversion: a foot is exactly .03048 dekameters.  But
the number given the other direction is inexact.

If you try to convert grains to pounds, you will see the following:

You have: grains
You want: pounds
* 0.00014285714
/ 7000

From the second line of the output you can immediately see that a grain
is equal to a seven thousandth of a pound.  This is not so obvious from
the first  line  of  the  output.   If  you  find   the  output  format
confusing, try using the '--verbose' option:

You have: grain
You want: aeginamina
grain = 0.00010416667 aeginamina
grain = (1 / 9600) aeginamina

If  you  request  a  conversion  between units which measure reciprocal
dimensions, then 'units' will display the conversion  results  with  an
extra note indicating that reciprocal conversion has been done:

You have: 6 ohms
You want: siemens
reciprocal conversion
* 0.16666667
/ 6

Reciprocal conversion can be suppressed by using the '--strict' option.
As usual, use the '--verbose' option to get more comprehensible output:

You have: tex
You want: typp
reciprocal conversion
1 / tex = 496.05465 typp
1 / tex = (1 / 0.0020159069) typp

You have: 20 mph
You want: sec/mile
reciprocal conversion
1 / 20 mph = 180 sec/mile
1 / 20 mph = (1 / 0.0055555556) sec/mile

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

If you only want to find the reduced form  or  definition  of  a  unit,
simply press return at the 'You want:' prompt.  Here is an example:

You have: jansky
You want:
Definition: fluxunit = 1e-26 W/m^2 Hz = 1e-26 kg / s^2

The  output  from  'units'  indicates  that the jansky is defined to be
equal to  a  fluxunit  which  in  turn  is  defined  to  be  a  certain
combination  of  watts,  meters,  and hertz.  The fully reduced (and in
this case somewhat more cryptic) form appears on the far right.

Some named units are  treated  as  dimensionless  in  some  situations.
These include the radian and steradian.  These units will be treated as
equal to 1 in units  conversions.   Power  is  equal  to  torque  times
angular  velocity.  This conversion can only be performed if the radian
is dimensionless.

You have: (14 ft lbf) (12 radians/sec)
You want: watts
* 227.77742
/ 0.0043902509

Note that named dimensionaless units are not treated  as  dimensionless
in  other  contexts.   They cannot be used as exponents so for example,
'meter^radian' is not allowed.

If you want a list of options you can  type  '?'  at  the  'You  want:'
prompt.   The  program  will  display  a  list of named units which are
conformable with the unit that you entered at the  'You  have:'  prompt
above.  Note that conformable unit combinations will not appear on this
list.

Typing 'help' at either prompt displays a short help message.  You  can
also  type 'help' followed by a unit name.  This will invoke a pager on
the units data base at the point where that unit is defined.   You  can
read  the  definition  and  comments  that  may  give  more  details or
historical information about the unit.

Typing 'search text' will display a list of  all  of  the  units  whose
names contain 'text' as a substring along with their definitions.  This
may help in the case where you aren't sure of the right unit name.

USING 'UNITS' NON-INTERACTIVELY
The 'units' program can  perform  units  conversions  non-interactively
from the command line.  To do this, type the command, type the original
units expression, and type the new units you want.  You  will  probably
need  to protect the units expressions from interpretation by the shell
using single quote characters.

If you type

units '2 liters' 'quarts'

then 'units' will print

* 2.1133764
/ 0.47317647

and then exit.  The output tells you that 2 liters is about 2.1 quarts,
or alternatively that a quart is about 0.47 times 2 liters.

If  the  conversion is successful, then 'units' will return success (0)
to the calling environment.  If 'units' is given non-conformable  units
to  convert,  it  will  print a message giving the reduced form of each
unit and it will return failure (nonzero) to the calling environment.

When 'units' is invoked with only one argument, it will print  out  the
definition  of  the specified unit.  It will return failure if the unit
is not defined and success if the unit is defined.

UNIT EXPRESSIONS
In order to enter more complicated units or fractions, you will need to
use  operations such as powers, products and division.  Powers of units
can be specified using the '^' character  as  shown  in  the  following
example, or by simple concatenation: 'cm3' is equivalent to 'cm^3'.  If
the exponent is more than one digit, the '^' is required.  An  exponent
like  '2^3^2'  is  evaluated  right  to left.  The '^' operator has the
second highest  precedence.   The  '**'  operator  is  provided  as  an
alternative exponent operator.

You have: cm^3
You want: gallons
* 0.00026417205
/ 3785.4118

You have: arabicfoot * arabictradepound * force
You want: ft lbf
* 0.7296
/ 1.370614

Multiplication  of  units  can  be  specified  by  using  spaces, or an
asterisk ('*').  If 'units' is invoked with the '--product' option then
the  hyphen  ('-') also acts as a multiplication operator.  Division of
units is indicated by the slash ('/') or by 'per'.

You have: furlongs per fortnight
You want: m/s
* 0.00016630986
/ 6012.8727

Historically, multiplication in units was assigned a higher  precedence
than  division.   This  disagrees with the usual precedence rules which
give multiplication and division equal precedence, and it  has  been  a
source of confusion for people who think of units as a calculator.

By  default,  multiplication  using  the  star  ('*')  now has the same
precedence as division and hence follows the  usual  precedence  rules.
If  units  is invoked with the the '--oldstar' option then then the old
behavior is activated and '*' will have  the  same  precedence  as  the
other multiplication operators described next.

Multiplication  using  a  space  or  using  the  hyphen  has  a  higher
precedence than division and is evaluated left to right.  So  @samp{m/s
s/day}  is equivalent to 'm / s s day' and has dimensions of length per
time cubed.  Similarly, '1/2 meter' refers  to  a  unit  of  reciprocal
length  equivalent  to  .5/meter,  which is probably not what you would
intend if you entered that expression.

You can indicate division of numbers with the vertical dash  ('|'),  so
if  you  wanted  half  a  meter you could write @samp{1|2 meter}.  This
operator has the highest precedence so the square root  of  two  thirds
could be written '2|3^1|2'.

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

Parentheses can be used for grouping as desired.

You have: (1/2) kg / (kg/meter)
You want: league
* 0.00010356166
/ 9656.0833

Prefixes  are  defined  separately  from  base  units.  In order to get
centimeters, the units database defines 'centi-' and 'c-' as  prefixes.
Prefixes  can  appear  alone  with no unit following them.  An exponent
applies only to the immediately preceding unit and its prefix  so  that
'cm^3' or 'centimeter^3' refer to cubic centimeters but 'centi*meter^3'
refers to hundredths of cubic meters.  Only one prefix is permitted per
unit, so 'micromicrofarad' will fail, but 'micro*microfarad' will work,
as will 'micro microfarad'..

For 'units', numbers are just another kind of unit.  They can appear as
many  times  as  you  like  and in any order in a unit expression.  For
example, to find the volume of a box which is 2 ft by 3 ft by 12 ft  in
steres, you could do the following:

You have: 2 ft 3 ft 12 ft
You want: stere
* 2.038813
/ 0.49048148

You have: \$ 5 / yard
You want: cents / inch
* 13.888889
/ 0.072

And  the  second  example  shows  how  the  dollar  sign  in  the units
conversion can precede the five.  Be careful:  'units'  will  interpret
'\$5' with no space as equivalent to dollars^5.

Outside  of  the  SI  system,  it  is  often desirable to add values of
different units together.  You may  also  wish  to  use  'units'  as  a
calculator  that  keeps  track of units.  Sums of conformable units are
written with the '+' character.

You have: 2 hours + 23 minutes + 32 seconds
You want: seconds
* 8612
/ 0.00011611705

You have: 12 ft + 3 in
You want: cm
* 373.38
/ 0.0026782366

You have: 2 btu + 450 ft lbf
You want: btu
* 2.5782804
/ 0.38785542

The expressions which are  added  together  must  reduce  to  identical
expressions in primitive units, or an error message will be displayed:

You have: 12 printerspoint + 4 heredium
^
Illegal sum of non-conformable units

Historically '-' has been used for products of units, which complicates
its iterpretation in 'units'.  Because 'units' provides  several  other
ways to obtain unit products, and because '-' is a subtraction operator
in general algebraic expressions, 'units' treats the binary  '-'  as  a
subtraction  operator  by  default.  This behavior can be altered using
the '--product' option which causes 'units' to  treat  the  binary  '-'
operator as a product operator.  Note that when '-' is a multiplication
operator it has  the  same  precedence  as  '*',  but  when  '-'  is  a
subtraction  operator  it  has  the  lower  precedence  as the addition
operator.

When  '-'  is  used  as  a  unary  operator  it  negates  its  operand.
Regardless  of  the  'units' options, if '-' appears after '(' or after
'+' then it will act as a negation operator.  So you can always compute
20 degrees minus 12 minutes by entering '20 degrees + -12 arcmin'.  You
must use this construction when you define new units because you cannot
know what options will be in force when your definition is processed.

The  '+' character sometimes appears in exponents like '3.43e+8'.  This
leads to an ambiguity in an expression like '3e+2 yC'.  The unit 'e' is
a  small  unit  of  charge,  so  this  can be regarded as equivalent to
'(3e+2) yC' or '(3 e)+(2 yC)'.  This ambiguity is  resolved  by  always
interpreting '+' as part of an exponent if possible.

Several  built  in  functions  are provided: 'sin', 'cos', 'tan', 'ln',
'log', 'log2', 'exp', 'acos', 'atan' and 'asin'.  The 'sin', 'cos', and
'tan'  functions require either a dimensionless argument or an argument
with dimensions of angle.

You have: sin(30 degrees)
You want:
Definition: 0.5

You have: sin(pi/2)
You want:
Definition: 1

You have: sin(3 kg)
^
Unit not dimensionless

The other functions on the list require dimensionless  arguments.   The
inverse  trigonometric  functions  return  arguments with dimensions of
angle.

If you wish to  take  roots  of  units,  you  may  use  the  'sqrt'  or
'cuberoot'  functions.   These functions require that the argument have
the  appropriate  root.   Higher  roots  can   be  obtained  by   using
fractional exponents:

You have: sqrt(acre)
You want: feet
* 208.71074
/ 0.0047913202

You have: (400 W/m^2 / stefanboltzmann)^(1/4)
You have:
Definition: 289.80882 K

You have: cuberoot(hectare)
^
Unit not a root

Nonlinear  units  are represented using functional notation.  They make
possible  nonlinear  unit  conversions  such  temperature.    This   is
different  from  the linear units that convert temperature differences.
Note the difference below.  The absolute  temperature  conversions  are
handled  by  units  starting  with  'temp', and you must use functional
notation.  The temperature differences are done  using  units  starting
with 'deg' and they do not require functional notation.

You have: tempF(45)
You want: tempC
7.2222222

You have: 45 degF
You want: degC
* 25
/ 0.04

Think of 'tempF(x)' not as a function but as a notation which indicates
that 'x' should have units of 'tempF' attached to  it.   See  Nonlinear
units.   The  first conversion shows that if it's 45 degrees Fahrehneit
outside it's 7.2 degrees Celsius.   The  second  conversions  indicates
that  a  change  of 45 degrees Fahrenheit corresponds to a change of 25
degrees Celsius.

Some other examples of nonlinears units are ring size and  wire  gauge.
There  are  numerous  different  gauges  and ring sizes.  See the units
database for more details.  Note that wire gauges with multiple  zeroes
are   signified   using  negative  numbers  where  two  zeroes  is  -1.
Alternatively, you can use the synonyms 'g00', 'g000', and so  on  that
are defined in the units database.

You have: wiregauge(11)
You want: inches
* 0.090742002
/ 11.020255

You have: brwiregauge(g00)
You want: inches
* 0.348
/ 2.8735632

You have: 1 mm
You want: wiregauge
18.201919

INVOKING 'UNITS'
You invoke 'units' like this:

units [OPTIONS] [FROM-UNIT [TO-UNIT]]

If  the  FROM-UNIT  and  TO-UNIT are omitted, then the program will use
interactive prompts to determine which  conversions  to  perform.   See
Interactive use.  If both FROM-UNIT and TO-UNIT are given, 'units' will
print the result of that single conversion  and  then  exit.   If  only
FROM-UNIT  appears  on  the  command  line,  'units'  will  display the
definition of that unit and exit.  Units specified on the command  line
will need to be quoted to protect them from shell interpretation and to
group them into two arguments.  See Command line use.

The following options allow you to read in an alternative  units  file,
check your units file, or change the output format:

-c, --check
Check that all units and prefixes defined in the units data file
reduce to primitive units.  Print  a  list  of  all  units  that
cannot  be  reduced.   Also display some other diagnostics about
suspicious definitions in the units data file.  Note  that  only
definitions active in the current locale are checked.

--check-verbose
Like  the  '-check'  option,  this option prints a list of units
that cannot be reduced.  But to help find unit  definitions that
cause endless loops, it lists the units as they are checked.  If
'units' hangs, then the last  unit  to  be  printed  has  a  bad
definition.   Note  that  only definitions active in the current
locale are checked.

-o format, --output-format format
Use the specified format for numeric output.  Format is the same
as  that  for  the  printf function in the ANSI C standard.  For
example, if you want more precision you might use '-o %.15g'.

-f filename, --file filename
Instruct  'units'  to  load  the  units  file  'filename'.    If
'filename'  is  the empty string ('-f "') then the default units
file will be loaded.  This enables you to load the default  file
plus  a  personal  units  file.   Up  to  25  units files may be
specified on  the  command  line.   This  option  overrides  the
'UNITSFILE' environment variable.

-h, --help
Print out a summary of the options for 'units'.

-m, --minus
Causes '-' to be interpreted as a subtraction operator.  This is
usually the default behavior.

-p, --product
Causes '-' to be interpreted as a multiplication  operator  when
it has two operands.  It will as a negation operator when it has
only one operand: '(-3)'.  Note that by default '-'  is  treated
as a subtraction operator.

,  --oldstar  Causes  '*' to have the old style precedence, higher than
the precedence of division so that '1/2*3' will equal '6'.

, --newstar Forces '*' to  have  the  new  (default)  precedence  which
follows  the  usual rules of algebra: the precedence of '*' is the same
as the precedence of '/', so that '1/2*3' will equal '3/2'.

, --compact Give compact output featuring only the  conversion  factor.
This turns off the '--verbose' option.

-q, --quiet, --silent
Suppress  prompting  of  the  user  for units and the display of
statistics about the number of units loaded.

-s, --strict
Suppress conversion of units to  their  reciprocal  units.   For
example,  'units' will normally convert hertz to seconds because
these units are reciprocals of each other.   The  strict  option
requires  that  units  be  strictly  conformable  to  perform  a
conversion, and will give an error if  you  attempt  to  convert
hertz to seconds.

-1, --one-line
Give  only  one line of output (the forward conversion).  Do not
print  the  reverse  conversion.   Note  that  if  a  reciprocal
conversion  is performed then 'units' will print still print the
"reciprocal conversion" line.

-t, --terse
Give terse output when converting units.   This  option  can  be
used  when  calling  'units'  from  another  program so that the
output is easy to parse.  This option has the combined effect of
these options:  '--strict' '--quiet' '--one-line' '--compact'.

-v, --verbose
Give  slightly  more verbose output when converting units.  When
combined with the '-c' option this  gives  the  same  effect  as
'--check-verbose'.

-V, --version
Print  program version number, tell whether the readline library
has been included, and give the location of  the  default  units
data file.

UNIT DEFINITIONS
The  conversion  information  is  read  from a units data file which is
called 'units.dat' and is probably located  in  the  '/usr/local/share'
directory.   If  you invoke 'units' with the '-V' option, it will print
the location of this file.  The default file includes  definitions  for
all  familiar  units,  abbreviations  and  metric  prefixes.   It  also
includes many obscure or archaic units.

Many constants of nature are defined, including these:

pi         ratio of circumference to diameter
c          speed of light
e          charge on an electron
force      acceleration of gravity
water      pressure per unit height of water
Hg         pressure per unit height of mercury
au         astronomical unit
k          Boltzman's constant

mu0        permeability of vacuum
epsilon0   permitivity of vacuum
G          gravitational constant
mach       speed of sound
The database includes  atomic  masses  for  all  of  the  elements  and
numerous  other  constants.  Also included are the densities of various
ingredients used in  baking  so  that  '2  cups  flour_sifted'  can  be
converted  to  'grams'.   This  is not an exhaustive list.  Consult the
units data file to see the complete list, or  to  see  the  definitions
that are used.

The  unit  'pound'  is  a  unit of mass.  To get force, multiply by the
force conversion unit 'force' or use the shorthand 'lbf'.   (Note  that
'g'  is  already taken as the standard abbreviation for the gram.)  The
unit 'ounce' is also a unit of mass.  The fluid ounce  is  'fluidounce'
or   'floz'.    British  capacity  units  that  differ  from  their  US
counterparts, such as the British Imperial gallon,  are  prefixed  with
'br'.   Currency  is  prefixed  with  its country name: 'belgiumfranc',
'britainpound'.

The US Survey foot, yard, and mile can be obtained by  using  the  'US'
prefix.   These  units  differ  slightly  from the international length
units.  They were in general use until 1959, and  are  still  used  for
geographic  surveys.  The acre is officially defined in terms of the US
Survey  foot.   If  you  want  an  acre  defined   according   to   the
international  foot, use 'intacre'.  The difference between these units
is about 4 parts  per  million.   The  British  also  used  a  slightly
different  length  measure before 1959.  These can be obtained with the
prefix 'UK'.

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'.  If that fails, 'units' will check for
a prefix.  All of the standard metric prefixes are defined.

To  find  out  what units and prefixes are available, read the standard
units data file.

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.  You can also add your  own  units  definitions  in  the
'.units.dat'  file  in  your home directory.  If this file exists it is
read before the units data file.  It will not  be  read  if  any  units
files are specified on the command line.

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.  A file can be  included  by  giving  the  command  '!include'
followed  by  the  file's  name.   The  file will be sought in the same
directory as the parent file unless a full path is given.

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.   Dimensionless  units  are  indicated  by using the string
'!dimensionless' for the unit definition.

When adding new units, be sure to use the '-c' option to check that the
new  units  reduce  properly.   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.

If  you  define any units which contain '+' characters, carefully check
them because the '-c' option will not catch non-conformable  sums.   Be
careful with the '-' operator as well.  When used as a binary operator,
the '-' character can perform addition or multiplication  depending  on
the  options used to invoke 'units'.  To ensure consistent behavior use
'-' only as a unary negation operator when writing  units  definitions.
To  multiply two units leave a space or use the '*' operator with care,
recalling that it has two possible precedence values  and  may  require
parentheses  to  ensure consistent behavior.  To compute the difference
of 'foo' and 'bar' write 'foo+(-bar)' or even 'foo+-bar'.

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
rad      !dimensionless   # 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'.

DEFINING NONLINEAR UNITS
Some  units  conversions  of  interest  are  nonlinear;  for   example,
temperature  conversions  between  the  Fahrenheit  and  Celsius scales
cannot be done by simply multiplying by conversions factors.

When you give a linear unit definition such as 'inch 2.54 cm'  you  are
providing  information  that  'units'  uses to convert values in inches
into primitive units of  meters.   For  nonlinear  units,  you  give  a
functional definition that provides the same information.

Nonlinear  units  are  represented  using a functional notation.  It is
best to regard this notation not as a function call but  as  a  way  of
adding  units to a number, much the same way that writing a linear unit
name after a number adds units to that number.   Internally,  nonlinear
units  are  defined  by  a  pair of functions which convert to and from
linear units in the data  file,  so  that  an  eventual  conversion  to
primitive units is possible.

Here is an example nonlinear unit definition:

tempF(x) [1;K] (x+(-32)) degF + stdtemp ; (tempF+(-stdtemp))/degF + 32

A  nonlinear  unit  definition comprises a unit name, a dummy parameter
name, two functions, and two corresponding units.  The  functions  tell
'units'  how  to convert to and from the new unit.  In order to produce
valid results, the arguments  of  these  functions  need  to  have  the
correct  dimensions.  To facilitate error checking, you may specify the
dimensions.

The definition begins with the unit name followed immediately (with  no
spaces)  by  a  '('  character.   In  parentheses  is  the  name of the
parameter.  Next is an optional specification of the units required  by
the  functions  in  this definition.  In the example above, the 'tempF'
function requires an input argument conformable with '1'.   For  normal
nonlinear  units  definitions  the  forward function will always take a
dimensionless  argument.   The  inverse  function  requires  an   input
argument  conformable  with  'K'.  In general the inverse function will
need units that match the quantity measured  by  your  nonlinear  unit.
The  sole  purpose  of  the expression in brackets to enable 'units' to
perform error checking on function arguments.

Next the function  definitions  appear.   In  the  example  above,  the
'tempF' function is defined by

tempF(x) = (x+(-32)) degF + stdtemp

This  gives  a  rule  for converting 'x' in the units 'tempF' to linear
units of absolute temperature, which makes it possible to convert  from
tempF to other units.

In  order  to  make conversions to Fahrenheit possible, you must give a
rule for the inverse conversions. The inverse will  be  'x(tempF)'  and
its  definition  appears  after  a  ';' character.  In our example, the
inverse is

x(tempF) = (tempF+(-stdtemp))/degF + 32

This inverse definition takes an absolute temperature as  its  argument
and  converts  it  to  the  Fahrenheit temperature.  The inverse can be
omitted by leaving out the ';' character, but then conversions  to  the
unit  will be impossible.  If the inverse is omitted then the '--check'
option will display a warning.  It is up to you to calculate and  enter
the  correct  inverse  function  to  obtain  proper  conversions.   The
'--check' option tests the inverse at one point and print an  error  if
it is not valid there, but this is not a guarantee that your inverse is
correct.

If you wish to make synonyms for nonlinear units,  you  still  need  to
define  both  the forward and inverse functions.  Inverse functions can
be obtained using the '~' operator.  So to create a synonym for 'tempF'
you could write

fahrenheit(x) [1;K] tempF(x); ~tempF(fahrenheit)

You  may occasionally wish to define a function that operates on units.
This can be done using a nonlinear unit definition.  For  example,  the
definition  below  provides conversion between radius and the area of a
circle.  Note that this definition  requires  a  length  as  input  and
produces  an  area  as  output,  as  indicated  by the specification in
brackets.

circlearea(r) [m;m^2] pi r^2 ; sqrt(circlearea/pi)

Sometimes you may be interested in a piecewise linear unit such as many
wire  gauges.   Piecewise  linear  units  can  be defined by specifying
conversions to linear units on a list of points.  Conversion  at  other
points  will  be done by linear interpolation.  A partial definition of
zinc gauge is

zincgauge[in] 1 0.002, 10 0.02, 15 0.04, 19 0.06, 23 0.1

In this example, 'zincgauge' is the name of the piecewise linear  unit.
The  definition  of  such  a  unit  is  indicated  by  the embedded '['
character.  After the bracket, you should  indicate  the  units  to  be
attached  to the numbers in the table.  No spaces can appear before the
']' character, so  a  definition  like  'foo[kg  meters]'  is  illegal;
instead write 'foo[kg*meters]'.  The definition of the unit consists of
a list of pairs optionally separated by commas.  This  list  defines  a
function for converting from the piecewise linear unit to linear units.
The first item in each pair is the function argument; the  second  item
is  the  value of the function at that argument (in the units specified
in brackets).  In this example, we define 'zincgauge' at  five  points.
For  example,  we  set 'zincgauge(1)' equal to '0.002 in'.  Definitions
like this  may  be   more  readable   if  written  using   continuation
characters as
zincgauge[in]         \
1        0.002 \
10        0.02 \
15        0.04 \
19        0.06 \
23        0.1

With  the  preceeding  definition,  the  following  conversion  can  be
performed:

You have: zincgauge(10)
You want: in
* 0.02
/ 50
You have: .01 inch
You want: zincgauge
5

If you define a piecewise linear unit that is not  strictly  monotonic,
then the inverse will not be well defined.  If the inverse is requested
for such a  unit,  'units'  will  return  the  smallest  inverse.   The
'--check'  option  will  print  a  warning if a non-monotonic piecewise
linear unit is encountered.

LOCALIZATION
Some  units  have  different  values  in  different   locations.    The
localization feature accomodates this by allowing the units database to
specify region dependent definitions.   A locale region  in  the  units
database begins with '!locale' followed by the name of the locale.  The
leading '!' must appear in the first column of the units database.  The
locale  region  is  terminated  by '!endlocale'.  The following example
shows how to define a couple units in a locale.

!locale en_GB
ton                     brton
gallon                  brgallon
!endlocale

The current locale is specified by the 'LOCALE'  environment  variable.
Note  that the '-c' option only checks the definitions which are active
for the current locale.

ENVIRONMENT VARIABLES
The 'units' programs uses the following environment variables.

LOCALE Specifies the locale.  The default is 'en_US'.  Sections of  the
units database are specific to certain locales.

PAGER  Specifies  the  pager  to  use  for  help and for displaying the
conformable units.  The help function browses the units database
and calls the pager using the '+nn' syntax for specifying a line
number.  The default pager is 'more', but  'less',  'emacs',  or
'vi' are possible alternatives.

UNITSFILE
Specifies  the  units  database  file  to  use  (instead  of the
default). This will be overridden by the '-f' option.  Note that
you  can  only  specify  a  single  units  database  using  this
environment variable.

If the 'readline' package has been compiled in, then  when  'units'  is
used   interactively,   numerous  command  line  editing  features  are
available.  To check if your version of 'units' includes the  readline,
invoke the program with the '--version' option.

For  complete information about readline, consult the documentation for
the readline package.  Without any configuration,  'units'  will  allow
editing  in the style of emacs.  Of particular use with 'units' are the
completion commands.

If you type a few characters and then hit 'ESC' followed by the '?' key
then  'units' will display a list of all the units which start with the
characters typed.  For example, if you type  'metr'  and  then  request
completion, you will see something like this:

You have: metr
metre             metriccup         metrichorsepower  metrictenth
metretes          metricfifth       metricounce       metricton
metriccarat       metricgrain       metricquart       metricyarncount
You have: metr

If  there  is  a unique way to complete a unitname, you can hit the tab
key and 'units' will provide the rest of the  unit  name.   If  'units'
beeps,  it  means that there is no unique completion.  Pressing the tab
key a second time will print the list of all completions.

FILES
/usr/share/misc/units.dat - the standard units data file

AUTHOR