How can you quickly get the complete path to a file for use in terminal?

readlink -f foo.bar

or (install it first)

realpath foo.bar

Just drag and drop the file in the terminal.

All good answers; Here is a tip for another situation.

If you are browsing your files using nautilus and you want the complete path of your current directory, then press CTRL+L. This changes the breadcrumb buttons temporarily back to the old-style address bar, allowing you to copy the path.