<title>Building on the Command Line</title>

    <title>Building from the Command Line</title>
    <para>
      It is often useful to test a build recipe, for instance before
      it is actually used by Hydra, when testing changes, or when
      debugging a build issue.  Since build recipes for Hydra jobsets
      are just plain Nix expressions, they can be evaluated using the
      standard Nix tools.
    </para>
    <para>
      To evaluate the <varname>tarball</varname> jobset of <xref
      linkend='ex-hello' />, just run:
      <screen>
	$ nix-build release.nix -A tarball
      </screen>
      However, doing this with <xref linkend='ex-hello' /> as is will
      probably yield an error like this:
      <screen>
	error: cannot auto-call a function that has an argument without a default value (`nixpkgs')
      </screen>
      This is because no value was specified for the
      <varname>nixpkgs</varname> argument of the Nix expression.
    </para>
    <para>
      This is fixed by providing a default value for that argument in
      the Nix expression, which will allow <command>nix-build</command>
      to auto-call the function: instfead of writing <literal>{ nixpkgs
      }:</literal>, we now write <literal>{ nixpkgs ? &lt;nixpkgs&gt;
      }:</literal>.  What it means is that, by default, the
      <varname>nixpkgs</varname> variable will be bound to the absolute
      path of any <filename>nixpkgs</filename> file found in the Nix
      search path.  Similarly, a default value for
      <varname>helloSrc</varname> needs to be provided.
    </para>

Overigens zijn die helemaal niet Hydra-specifiek, je kunt ze gewoon vanaf de
command line bouwen, bijv. als je een patchelf checkout hebt (met een nixpkgs
checkout in ../nixpkgs):

    <para>
      Thus, assuming a checkout of Nixpkgs is available under
      <filename>$HOME/src/nixpkgs</filename>, the
      <varname>tarball</varname> jobset can now be evaluated by running:

<screen>
$ nix-build release.nix -A rpm_fedora10i386
</screen>

      <screen>
	$ nix-build -I ~/src release.nix -A tarball
      </screen>
      Similarly, the <varname>build</varname> jobset can be evaluated:
      <screen>
	$ nix-build -I ~/src release.nix -A build
      </screen>
      The <varname>build</varname> job reuses the result of the
      <varname>tarball</varname> job, rebuilding it only if it needs to.
    </para>