getEnv returns the value of the environment variable s, or an empty string if the variable doesn't exist. This function should be used with care, as it can introduce all sorts of nasty environment dependencies in your Nix expression.

getEnv is used in nixpkgs for evil impurities such as locating the file ~/.config/nixpkgs/config.nix which contains user-local settings for nixpkgs. (That is, it does a getEnv "HOME" to locate the user's home directory.)

When in pure evaluation mode, this function always returns an empty string.