builtins.toFile

Takes 2 arguments

name, s

Store the string s in a file in the Nix store and return its path. The file has suffix name. This file can be used as an input to derivations. One application is to write builders “inline”. For instance, the following Nix expression combines the Nix expression for GNU Hello and its build script into one file:

{ stdenv, fetchurl, perl }:

stdenv.mkDerivation {
  name = "hello-2.1.1";

  builder = builtins.toFile "builder.sh" "
    source $stdenv/setup

    PATH=$perl/bin:$PATH

    tar xvfz $src
    cd hello-*
    ./configure --prefix=$out
    make
    make install
  ";

  src = fetchurl {
    url = "http://ftp.nluug.nl/pub/gnu/hello/hello-2.1.1.tar.gz";
    sha256 = "1md7jsfd8pa45z73bz1kszpp01yw6x5ljkjk2hx7wl800any6465";
  };
  inherit perl;
}

It is even possible for one file to refer to another, e.g.,

builder = let
  configFile = builtins.toFile "foo.conf" "
    # This is some dummy configuration file.
    ...
  ";
in builtins.toFile "builder.sh" "
  source $stdenv/setup
  ...
  cp ${configFile} $out/etc/foo.conf
";

Note that ${configFile} is a string interpolation, so the result of the expression configFile (i.e., a path like /nix/store/m7p7jfny445k...-foo.conf) will be spliced into the resulting string.

It is however not allowed to have files mutually referring to each other, like so:

let
  foo = builtins.toFile "foo" "...${bar}...";
  bar = builtins.toFile "bar" "...${foo}...";
in foo

This is not allowed because it would cause a cyclic dependency in the computation of the cryptographic hashes for foo and bar.

It is also not possible to reference the result of a derivation. If you are using Nixpkgs, the writeTextFile function is able to do that.

Noogle also knows

toFile :: Path -> String -> StorePath