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.