73 lines
3.2 KiB
Markdown
73 lines
3.2 KiB
Markdown
|
# Build Script
|
||
|
|
|
||
|
|
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.
|