The nixpkgs repository has several utility functions to manipulate Nix expressions.

This function inside the nixpkgs expression (pkgs) can be used to override the set of packages itself. Warning: this function is expensive and must not be used from within the nixpkgs repository. Example usage: let pkgs = import <nixpkgs> {}; newpkgs = pkgs.overridePackages (self: super: { foo = super.foo.override { ... }; }; in ... The resulting newpkgs will have the new foo expression, and all other expressions depending on foo will also use the new foo expression. The behavior of this function is similar to config.packageOverrides. The self parameter refers to the final package set with the applied overrides. Using this parameter may lead to infinite recursion if not used consciously. The super parameter refers to the old package set. It's equivalent to pkgs in the above example.

The function override is usually available for all the derivations in the nixpkgs expression (pkgs). It is used to override the arguments passed to a function. Example usages: pkgs.foo.override { arg1 = val1; arg2 = val2; ... } pkgs.overridePackages (self: super: { foo = super.foo.override { barSupport = true ; }; }) mypkg = pkgs.callPackage ./mypkg.nix { mydep = pkgs.mydep.override { ... }; }) In the first example, pkgs.foo is the result of a function call with some default arguments, usually a derivation. Using pkgs.foo.override will call the same function with the given new arguments.

The function overrideDerivation is usually available for all the derivations in the nixpkgs expression (pkgs). It is used to create a new derivation by overriding the attributes of the original derivation according to the given function. Example usage: mySed = pkgs.gnused.overrideDerivation (oldAttrs: { name = "sed-4.2.2-pre"; src = fetchurl { url = ftp://alpha.gnu.org/gnu/sed/sed-4.2.2-pre.tar.bz2; sha256 = "11nq06d131y4wmf3drm0yk502d2xc6n5qy82cg88rb9nqd2lj41k"; }; patches = []; }); In the above example, the name, src and patches of the derivation will be overridden, while all other attributes will be retained from the original derivation. The argument oldAttrs is used to refer to the attribute set of the original derivation.

The function lib.makeOverridable is used make the result of a function easily customizable. This utility only makes sense for functions that accept an argument set and return an attribute set. Example usage: f = { a, b }: { result = a+b; } c = lib.makeOverridable f { a = 1; b = 2; } The variable c is the value of the f function applied with some default arguments. Hence the value of c.result is 3, in this example. The variable c however also has some additional functions, like c.override which can be used to override the default arguments. In this example the value of (c.override { a = 4; }).result is 6.

buildFHSChrootEnv and buildFHSUserEnv provide a way to build and run FHS-compatible lightweight sandboxes. They get their own isolated root with binded /nix/store, so their footprint in terms of disk space needed is quite small. This allows one to run software which is hard or unfeasible to patch for NixOS -- 3rd-party source trees with FHS assumptions, games distributed as tarballs, software with integrity checking and/or external self-updated binaries. buildFHSChrootEnv allows to create persistent environments, which can be constructed, deconstructed and entered by multiple users at once. A downside is that it requires root access for both those who create and destroy and those who enter it. It can be useful to create environments for daemons that one can enter and observe. buildFHSUserEnv uses Linux namespaces feature to create temporary lightweight environments which are destroyed after all child processes exit. It does not require root access, and can be useful to create sandboxes and wrap applications. Those functions both rely on buildFHSEnv, which creates an actual directory structure given a list of necessary packages and extra build commands. buildFHSChrootEnv and buildFHSUserEnv both accept those arguments which are passed to buildFHSEnv: name Environment name. targetPkgs Packages to be installed for the main host's architecture (i.e. x86_64 on x86_64 installations). multiPkgs Packages to be installed for all architectures supported by a host (i.e. i686 and x86_64 on x86_64 installations). extraBuildCommands Additional commands to be executed for finalizing the directory structure. extraBuildCommandsMulti Like extraBuildCommandsMulti, but executed only on multilib architectures. Additionally, buildFHSUserEnv accepts runScript parameter, which is a command that would be executed inside the sandbox and passed all the command line arguments. It default to bash. One can create a simple environment using a shell.nix like that: {} }: (pkgs.buildFHSUserEnv { name = "simple-x11-env"; targetPkgs = pkgs: (with pkgs; [ udev alsaLib ]) ++ (with pkgs.xorg; [ libX11 libXcursor libXrandr ]); multiPkgs = pkgs: (with pkgs; [ udev alsaLib ]) ++ (with []; runScript = "bash"; }).env ]]> Running nix-shell would then drop you into a shell with these libraries and binaries available. You can use this to run closed-source applications which expect FHS structure without hassles: simply change runScript to the application path, e.g. ./bin/start.sh -- relative paths are supported.