nix instantiate
    example.nix
    is the same as

    -E|--expr 'import ./example.nix'

    --eval example.nix
    --eval --expr '2 + 2'
    --eval --expr 'import ./example.nix'

    --read-write-mode
        ???

    -I or $NIX_PATH
        nix path
        note: -I accepts a single directory
        The NIX_PATH also accepts a different yet very handy syntax: "somename=somepath".
        That is, instead of searching inside a directory for a name, we specify exactly the value of that name.
            $ NIX_PATH="ping=/bin/ping" nix-instantiate --eval -E '<ping>'