Can GNU units or anything like it accept a string such as "100F to C" and understand what I'd like? If so I'd love to use that instead.

Temperature conversions are a little goofy because they are nonlinear. Google's 100F to C is:

  dfc@ronin:~$ units
  2866 units, 109 prefixes, 79 nonlinear units
  
  You have: tempF(100)
  You want: tempC
          37.777778

The full explanation is here: https://www.gnu.org/software/units/manual/units.html#Tempera...

I followed the like because I thought maybe i've been doimg temperature conversioms wrong all my life. Temperature conversions are so linear. They need to use a more correct term — e.g. simply proportional.

Having to add or substract a constant before or after scaling does not make a relationship non-linear.

This may be getting pedantic, but the relationship is affine not linear - linear would mean k times the original input would yield k times the original output.

It does not convert currencies like Google.