How to Set or Get Environment in Ex mode in Vim

It is quite often that you need to pass data from to Vim to shell[e.g. bash]. Here is the command. We want to pass the full file name in current buffer to an Environment variable, e.g pp

:let $pp=expand('%:p')
:             => Ex mode in Vim, nothing fancy
let           => Set the internal variable to the result of right handside
$             => It indicates the 'Environment' variable
expand('%:p') => It is just an expression

How to use $pp in shell? e.g.
:sh  => drop into your shell
>find ./ -iname $pp -print

Note: $pp is only available in your current session.