Like many languages Nix has a REPL, where REPL stands for "read eval print loop". The REPL is convenient tool that reads a nix-expression and prints the evaluated result.
You can install it:
nix-env -iA nixpkgs.nix-repl
Now test that it works by running nix-repl and entering the simple
expression 1 + 1:
$ nix-repl
Welcome to Nix version 1.10. Type :? for help.
nix-repl> 1 + 1
2