Generic Builder Syntax

Recall that the build script for GNU Hello looked something like this:

PATH=$perl/bin:$PATH
tar xvfz $src
cd hello-*
./configure --prefix=$out
make
make install

The builders for almost all Unix packages look like this — set up some environment variables, unpack the sources, configure, build, and install. For this reason the standard environment provides some Bash functions that automate the build process. Here is what a builder using the generic build facilities looks like:

buildInputs="$perl" ①

source $stdenv/setup ②

genericBuild ③

Here is what each line means:

  1. The buildInputs variable tells setup to use the indicated packages as “inputs”. This means that if a package provides a bin subdirectory, it's added to PATH; if it has a include subdirectory, it's added to GCC's header search path; and so on. (This is implemented in a modular way: setup tries to source the file pkg/nix-support/setup-hook of all dependencies. These “setup hooks” can then set up whatever environment variables they want; for instance, the setup hook for Perl sets the PERL5LIB environment variable to contain the lib/site_perl directories of all inputs.)

  2. The function genericBuild is defined in the file $stdenv/setup.

  3. The final step calls the shell function genericBuild, which performs the steps that were done explicitly in the previous build script. The generic builder is smart enough to figure out whether to unpack the sources using gzip, bzip2, etc. It can be customised in many ways; see the Nixpkgs manual for details.

Discerning readers will note that the buildInputs could just as well have been set in the Nix expression, like this:

  buildInputs = [ perl ];

The perl attribute can then be removed, and the builder becomes even shorter:

source $stdenv/setup
genericBuild

In fact, mkDerivation provides a default builder that looks exactly like that, so it is actually possible to omit the builder for Hello entirely.