# Functions reference

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

## pkgs.overridePackages

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.

## <pkg>.override

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.

## <pkg>.overrideDerivation

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.

## lib.makeOverridable

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/buildFHSUserEnv

`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 ? import <nixpkgs> {} }:

    (pkgs.buildFHSUserEnv {
      name = "simple-x11-env";
      targetPkgs = pkgs: (with pkgs;
        [ udev
          alsaLib
        ]) ++ (with pkgs.xlibs;
        [ 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.