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70 lines
3 KiB
Markdown
70 lines
3 KiB
Markdown
# Build Script
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Here is the builder referenced from Hello's Nix expression (stored in
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`pkgs/applications/misc/hello/ex-1/builder.sh`):
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```bash
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source $stdenv/setup ①
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PATH=$perl/bin:$PATH ②
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tar xvfz $src ③
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cd hello-*
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./configure --prefix=$out ④
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make ⑤
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make install
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```
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The builder can actually be made a lot shorter by using the *generic
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builder* functions provided by `stdenv`, but here we write out the build
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steps to elucidate what a builder does. It performs the following steps:
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1. When Nix runs a builder, it initially completely clears the
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environment (except for the attributes declared in the derivation).
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This is done to prevent undeclared inputs from being used in the
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build process. If for example the `PATH` contained `/usr/bin`, then
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you might accidentally use `/usr/bin/gcc`.
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So the first step is to set up the environment. This is done by
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calling the `setup` script of the standard environment. The
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environment variable `stdenv` points to the location of the
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standard environment being used. (It wasn't specified explicitly
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as an attribute in Hello's Nix expression, but `mkDerivation` adds
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it automatically.)
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2. Since Hello needs Perl, we have to make sure that Perl is in the
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`PATH`. The `perl` environment variable points to the location of
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the Perl package (since it was passed in as an attribute to the
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derivation), so `$perl/bin` is the directory containing the Perl
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interpreter.
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3. Now we have to unpack the sources. The `src` attribute was bound to
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the result of fetching the Hello source tarball from the network, so
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the `src` environment variable points to the location in the Nix
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store to which the tarball was downloaded. After unpacking, we `cd`
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to the resulting source directory.
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The whole build is performed in a temporary directory created in
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`/tmp`, by the way. This directory is removed after the builder
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finishes, so there is no need to clean up the sources afterwards.
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Also, the temporary directory is always newly created, so you don't
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have to worry about files from previous builds interfering with the
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current build.
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4. GNU Hello is a typical Autoconf-based package, so we first have to
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run its `configure` script. In Nix every package is stored in a
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separate location in the Nix store, for instance
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`/nix/store/9a54ba97fb71b65fda531012d0443ce2-hello-2.1.1`. Nix
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computes this path by cryptographically hashing all attributes of
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the derivation. The path is passed to the builder through the `out`
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environment variable. So here we give `configure` the parameter
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`--prefix=$out` to cause Hello to be installed in the expected
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location.
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5. Finally we build Hello (`make`) and install it into the location
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specified by `out` (`make install`).
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If you are wondering about the absence of error checking on the result
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of various commands called in the builder: this is because the shell
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script is evaluated with Bash's `-e` option, which causes the script to
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be aborted if any command fails without an error check.
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