Merge pull request #5212 from mkenigs/auto-uid-allocation

Merge master into #3600

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+# Generic Builder Syntax
+
+Recall that the [build script for GNU Hello](build-script.md) looked
+something like this:
+
+```bash
+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:
+
+```bash
+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:
+
+```nix
+  buildInputs = [ perl ];
+```
+
+The `perl` attribute can then be removed, and the builder becomes even
+shorter:
+
+```bash
+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.