This chapter shows you how to write Nix expressions, which instruct Nix how to build packages. It starts with a simple example (a Nix expression for GNU Hello), and then moves on to a more in-depth look at the Nix expression language. This chapter is mostly about the Nix expression language. For more extensive information on adding packages to the Nix Packages collection (such as functions in the standard environment and coding conventions), please consult its manual.