84 lines
3.7 KiB
XML
84 lines
3.7 KiB
XML
<section xmlns="http://docbook.org/ns/docbook"
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xmlns:xlink="http://www.w3.org/1999/xlink"
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xmlns:xi="http://www.w3.org/2001/XInclude"
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version="5.0"
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xml:id='sec-building-simple'>
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<title>Building and Testing</title>
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<para>You can now try to build Hello. Of course, you could do
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<literal>nix-env -i hello</literal>, but you may not want to install a
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possibly broken package just yet. The best way to test the package is by
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using the command <command linkend="sec-nix-build">nix-build</command>,
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which builds a Nix expression and creates a symlink named
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<filename>result</filename> in the current directory:
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<screen>
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$ nix-build -A hello
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building path `/nix/store/632d2b22514d...-hello-2.1.1'
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hello-2.1.1/
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hello-2.1.1/intl/
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hello-2.1.1/intl/ChangeLog
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<replaceable>...</replaceable>
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$ ls -l result
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lrwxrwxrwx ... 2006-09-29 10:43 result -> /nix/store/632d2b22514d...-hello-2.1.1
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$ ./result/bin/hello
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Hello, world!</screen>
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The <link linkend='opt-attr'><option>-A</option></link> option selects
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the <literal>hello</literal> attribute. This is faster than using the
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symbolic package name specified by the <literal>name</literal>
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attribute (which also happens to be <literal>hello</literal>) and is
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unambiguous (there can be multiple packages with the symbolic name
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<literal>hello</literal>, but there can be only one attribute in a set
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named <literal>hello</literal>).</para>
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<para><command>nix-build</command> registers the
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<filename>./result</filename> symlink as a garbage collection root, so
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unless and until you delete the <filename>./result</filename> symlink,
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the output of the build will be safely kept on your system. You can
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use <command>nix-build</command>’s <option
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linkend='opt-out-link'>-o</option> switch to give the symlink another
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name.</para>
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<para>Nix has a transactional semantics. Once a build finishes
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successfully, Nix makes a note of this in its database: it registers
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that the path denoted by <envar>out</envar> is now
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<quote>valid</quote>. If you try to build the derivation again, Nix
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will see that the path is already valid and finish immediately. If a
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build fails, either because it returns a non-zero exit code, because
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Nix or the builder are killed, or because the machine crashes, then
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the output paths will not be registered as valid. If you try to build
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the derivation again, Nix will remove the output paths if they exist
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(e.g., because the builder died half-way through <literal>make
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install</literal>) and try again. Note that there is no
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<quote>negative caching</quote>: Nix doesn't remember that a build
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failed, and so a failed build can always be repeated. This is because
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Nix cannot distinguish between permanent failures (e.g., a compiler
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error due to a syntax error in the source) and transient failures
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(e.g., a disk full condition).</para>
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<para>Nix also performs locking. If you run multiple Nix builds
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simultaneously, and they try to build the same derivation, the first
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Nix instance that gets there will perform the build, while the others
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block (or perform other derivations if available) until the build
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finishes:
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<screen>
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$ nix-build -A hello
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waiting for lock on `/nix/store/0h5b7hp8d4hqfrw8igvx97x1xawrjnac-hello-2.1.1x'</screen>
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So it is always safe to run multiple instances of Nix in parallel
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(which isn’t the case with, say, <command>make</command>).</para>
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<para>If you have a system with multiple CPUs, you may want to have
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Nix build different derivations in parallel (insofar as possible).
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Just pass the option <link linkend='opt-max-jobs'><option>-j
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<replaceable>N</replaceable></option></link>, where
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<replaceable>N</replaceable> is the maximum number of jobs to be run
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in parallel, or set. Typically this should be the number of
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CPUs.</para>
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</section>
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