defining real time variables for a shell

[Eric Smith <es@xxxxxxxxxxxx>]

Hi zshellers

What would be a good way to define in a shell a variable for say currencies
like I want to define $USD as 1.33 and $GBP as 1.44 for example.

These values need to be updated at least daily and must be
available to shells that are already open (I use screen and am
lazy). 

I am thinking maybe to use a preexec function that parses a file
with the values.

Thanks!

-- 
Eric Smith