How can I execute a Vim selection or range in bash without replacing it?

Type the :w command like this:

:w !command

As you type it, it will appear like this:

:'<,'>w !command

See

:help :w_c

In vim 8.0, you can select a range of lines and run :terminal bash. This will open a terminal running bash with your selection as stdin.

:terminal node, :terminal ruby, :terminal python seem to work nicely as well.