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doc/manual/src/expressions/builder-syntax.md

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+# Builder Syntax
+
+    source $stdenv/setup 
+    
+    PATH=$perl/bin:$PATH 
+    
+    tar xvfz $src 
+    cd hello-*
+    ./configure --prefix=$out 
+    make 
+    make install
+
+[example\_title](#ex-hello-builder) shows the builder referenced from
+Hello's Nix expression (stored in
+`pkgs/applications/misc/hello/ex-1/builder.sh`). The builder can
+actually be made a lot shorter by using the *generic builder* functions
+provided by `stdenv`, but here we write out the build steps to elucidate
+what a builder does. It performs the following steps:
+
+  - When Nix runs a builder, it initially completely clears the
+    environment (except for the attributes declared in the derivation).
+    For instance, the PATH variable is empty\[1\]. This is done to
+    prevent undeclared inputs from being used in the build process. If
+    for example the PATH contained `/usr/bin`, then you might
+    accidentally use `/usr/bin/gcc`.
+    
+    So the first step is to set up the environment. This is done by
+    calling the `setup` script of the standard environment. The
+    environment variable stdenv points to the location of the standard
+    environment being used. (It wasn't specified explicitly as an
+    attribute in [???](#ex-hello-nix), but `mkDerivation` adds it
+    automatically.)
+
+  - Since Hello needs Perl, we have to make sure that Perl is in the
+    PATH. The perl environment variable points to the location of the
+    Perl package (since it was passed in as an attribute to the
+    derivation), so `$perl/bin` is the directory containing the Perl
+    interpreter.
+
+  - Now we have to unpack the sources. The `src` attribute was bound to
+    the result of fetching the Hello source tarball from the network, so
+    the src environment variable points to the location in the Nix store
+    to which the tarball was downloaded. After unpacking, we `cd` to the
+    resulting source directory.
+    
+    The whole build is performed in a temporary directory created in
+    `/tmp`, by the way. This directory is removed after the builder
+    finishes, so there is no need to clean up the sources afterwards.
+    Also, the temporary directory is always newly created, so you don't
+    have to worry about files from previous builds interfering with the
+    current build.
+
+  - GNU Hello is a typical Autoconf-based package, so we first have to
+    run its `configure` script. In Nix every package is stored in a
+    separate location in the Nix store, for instance
+    `/nix/store/9a54ba97fb71b65fda531012d0443ce2-hello-2.1.1`. Nix
+    computes this path by cryptographically hashing all attributes of
+    the derivation. The path is passed to the builder through the out
+    environment variable. So here we give `configure` the parameter
+    `--prefix=$out` to cause Hello to be installed in the expected
+    location.
+
+  - Finally we build Hello (`make`) and install it into the location
+    specified by out (`make install`).
+
+If you are wondering about the absence of error checking on the result
+of various commands called in the builder: this is because the shell
+script is evaluated with Bash's `-e` option, which causes the script to
+be aborted if any command fails without an error check.
+
+1.  Actually, it's initialised to `/path-not-set` to prevent Bash from
+    setting it to a default value.