3.2 KiB
Build Script
source $stdenv/setup
PATH=$perl/bin:$PATH
tar xvfz $src
cd hello-*
./configure --prefix=$out
make
make install
example_title 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 thePATH
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 variablestdenv
points to the location of the standard environment being used. (It wasn't specified explicitly as an attribute in ???, butmkDerivation
adds it automatically.) -
Since Hello needs Perl, we have to make sure that Perl is in the
PATH
. Theperl
environment variable points to the location of the Perl package (since it was passed in as an attribute to the derivation), so$perl/bin
is the directory containing the Perl interpreter. -
Now we have to unpack the sources. The
src
attribute was bound to the result of fetching the Hello source tarball from the network, so thesrc
environment variable points to the location in the Nix store to which the tarball was downloaded. After unpacking, wecd
to the resulting source directory.The whole build is performed in a temporary directory created in
/tmp
, by the way. This directory is removed after the builder finishes, so there is no need to clean up the sources afterwards. Also, the temporary directory is always newly created, so you don't have to worry about files from previous builds interfering with the current build. -
GNU Hello is a typical Autoconf-based package, so we first have to run its
configure
script. In Nix every package is stored in a separate location in the Nix store, for instance/nix/store/9a54ba97fb71b65fda531012d0443ce2-hello-2.1.1
. Nix computes this path by cryptographically hashing all attributes of the derivation. The path is passed to the builder through theout
environment variable. So here we giveconfigure
the parameter--prefix=$out
to cause Hello to be installed in the expected location. -
Finally we build Hello (
make
) and install it into the location specified byout
(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.
- Actually, it's initialised to
/path-not-set
to prevent Bash from setting it to a default value.