<chapter xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id='chap-writing-nix-expressions' xmlns:xi="http://www.w3.org/2001/XInclude"> <title>Writing Nix Expressions</title> <para>This chapter shows you how to write Nix expressions, which are the things that tell Nix how to build packages. It starts with a simple example (a Nix expression for GNU Hello), and then moves on to a more in-depth look at the Nix expression language.</para> <note><para>This chapter is mostly about the Nix expression language. For more extensive information on adding packages to the Nix Packages collection (such as functions in the standard environment and coding conventions), please consult <link xlink:href="http://nixos.org/nixpkgs/manual/">its manual</link>.</para></note> <section><title>A simple Nix expression</title> <para>This section shows how to add and test the <link xlink:href='http://www.gnu.org/software/hello/hello.html'>GNU Hello package</link> to the Nix Packages collection. Hello is a program that prints out the text <quote>Hello, world!</quote>.</para> <para>To add a package to the Nix Packages collection, you generally need to do three things: <orderedlist> <listitem><para>Write a Nix expression for the package. This is a file that describes all the inputs involved in building the package, such as dependencies, sources, and so on.</para></listitem> <listitem><para>Write a <emphasis>builder</emphasis>. This is a shell script<footnote><para>In fact, it can be written in any language, but typically it's a <command>bash</command> shell script.</para></footnote> that actually builds the package from the inputs.</para></listitem> <listitem><para>Add the package to the file <filename>pkgs/top-level/all-packages.nix</filename>. The Nix expression written in the first step is a <emphasis>function</emphasis>; it requires other packages in order to build it. In this step you put it all together, i.e., you call the function with the right arguments to build the actual package.</para></listitem> </orderedlist> </para> <section><title>The Nix expression</title> <example xml:id='ex-hello-nix'><title>Nix expression for GNU Hello (<filename>default.nix</filename>)</title> <programlisting> { stdenv, fetchurl, perl }: <co xml:id='ex-hello-nix-co-1' /> stdenv.mkDerivation { <co xml:id='ex-hello-nix-co-2' /> name = "hello-2.1.1"; <co xml:id='ex-hello-nix-co-3' /> builder = ./builder.sh; <co xml:id='ex-hello-nix-co-4' /> src = fetchurl { <co xml:id='ex-hello-nix-co-5' /> url = ftp://ftp.nluug.nl/pub/gnu/hello/hello-2.1.1.tar.gz; md5 = "70c9ccf9fac07f762c24f2df2290784d"; }; inherit perl; <co xml:id='ex-hello-nix-co-6' /> }</programlisting> </example> <para><xref linkend='ex-hello-nix' /> shows a Nix expression for GNU Hello. It's actually already in the Nix Packages collection in <filename>pkgs/applications/misc/hello/ex-1/default.nix</filename>. It is customary to place each package in a separate directory and call the single Nix expression in that directory <filename>default.nix</filename>. The file has the following elements (referenced from the figure by number): <calloutlist> <callout arearefs='ex-hello-nix-co-1'> <para>This states that the expression is a <emphasis>function</emphasis> that expects to be called with three arguments: <varname>stdenv</varname>, <varname>fetchurl</varname>, and <varname>perl</varname>. They are needed to build Hello, but we don't know how to build them here; that's why they are function arguments. <varname>stdenv</varname> is a package that is used by almost all Nix Packages packages; it provides a <quote>standard</quote> environment consisting of the things you would expect in a basic Unix environment: a C/C++ compiler (GCC, to be precise), the Bash shell, fundamental Unix tools such as <command>cp</command>, <command>grep</command>, <command>tar</command>, etc. <varname>fetchurl</varname> is a function that downloads files. <varname>perl</varname> is the Perl interpreter.</para> <para>Nix functions generally have the form <literal>{ x, y, ..., z }: e</literal> where <varname>x</varname>, <varname>y</varname>, etc. are the names of the expected arguments, and where <replaceable>e</replaceable> is the body of the function. So here, the entire remainder of the file is the body of the function; when given the required arguments, the body should describe how to build an instance of the Hello package.</para> </callout> <callout arearefs='ex-hello-nix-co-2'> <para>So we have to build a package. Building something from other stuff is called a <emphasis>derivation</emphasis> in Nix (as opposed to sources, which are built by humans instead of computers). We perform a derivation by calling <varname>stdenv.mkDerivation</varname>. <varname>mkDerivation</varname> is a function provided by <varname>stdenv</varname> that builds a package from a set of <emphasis>attributes</emphasis>. A set is just a list of key/value pairs where each key is a string and each value is an arbitrary Nix expression. They take the general form <literal>{ <replaceable>name1</replaceable> = <replaceable>expr1</replaceable>; <replaceable>...</replaceable> <replaceable>nameN</replaceable> = <replaceable>exprN</replaceable>; }</literal>.</para> </callout> <callout arearefs='ex-hello-nix-co-3'> <para>The attribute <varname>name</varname> specifies the symbolic name and version of the package. Nix doesn't really care about these things, but they are used by for instance <command>nix-env -q</command> to show a <quote>human-readable</quote> name for packages. This attribute is required by <varname>mkDerivation</varname>.</para> </callout> <callout arearefs='ex-hello-nix-co-4'> <para>The attribute <varname>builder</varname> specifies the builder. This attribute can sometimes be omitted, in which case <varname>mkDerivation</varname> will fill in a default builder (which does a <literal>configure; make; make install</literal>, in essence). Hello is sufficiently simple that the default builder would suffice, but in this case, we will show an actual builder for educational purposes. The value <command>./builder.sh</command> refers to the shell script shown in <xref linkend='ex-hello-builder' />, discussed below.</para> </callout> <callout arearefs='ex-hello-nix-co-5'> <para>The builder has to know what the sources of the package are. Here, the attribute <varname>src</varname> is bound to the result of a call to the <command>fetchurl</command> function. Given a URL and an MD5 hash of the expected contents of the file at that URL, this function builds a derivation that downloads the file and checks its hash. So the sources are a dependency that like all other dependencies is built before Hello itself is built.</para> <para>Instead of <varname>src</varname> any other name could have been used, and in fact there can be any number of sources (bound to different attributes). However, <varname>src</varname> is customary, and it's also expected by the default builder (which we don't use in this example).</para> </callout> <callout arearefs='ex-hello-nix-co-6'> <para>Since the derivation requires Perl, we have to pass the value of the <varname>perl</varname> function argument to the builder. All attributes in the set are actually passed as environment variables to the builder, so declaring an attribute <programlisting> perl = perl;</programlisting> will do the trick: it binds an attribute <varname>perl</varname> to the function argument which also happens to be called <varname>perl</varname>. However, it looks a bit silly, so there is a shorter syntax. The <literal>inherit</literal> keyword causes the specified attributes to be bound to whatever variables with the same name happen to be in scope.</para> </callout> </calloutlist> </para> </section> <section><title>The builder</title> <example xml:id='ex-hello-builder'><title>Build script for GNU Hello (<filename>builder.sh</filename>)</title> <programlisting> source $stdenv/setup <co xml:id='ex-hello-builder-co-1' /> PATH=$perl/bin:$PATH <co xml:id='ex-hello-builder-co-2' /> tar xvfz $src <co xml:id='ex-hello-builder-co-3' /> cd hello-* ./configure --prefix=$out <co xml:id='ex-hello-builder-co-4' /> make <co xml:id='ex-hello-builder-co-5' /> make install</programlisting> </example> <para><xref linkend='ex-hello-builder' /> shows the builder referenced from Hello's Nix expression (stored in <filename>pkgs/applications/misc/hello/ex-1/builder.sh</filename>). The builder can actually be made a lot shorter by using the <emphasis>generic builder</emphasis> functions provided by <varname>stdenv</varname>, but here we write out the build steps to elucidate what a builder does. It performs the following steps:</para> <calloutlist> <callout arearefs='ex-hello-builder-co-1'> <para>When Nix runs a builder, it initially completely clears the environment (except for the attributes declared in the derivation). For instance, the <envar>PATH</envar> variable is empty<footnote><para>Actually, it's initialised to <filename>/path-not-set</filename> to prevent Bash from setting it to a default value.</para></footnote>. This is done to prevent undeclared inputs from being used in the build process. If for example the <envar>PATH</envar> contained <filename>/usr/bin</filename>, then you might accidentally use <filename>/usr/bin/gcc</filename>.</para> <para>So the first step is to set up the environment. This is done by calling the <filename>setup</filename> script of the standard environment. The environment variable <envar>stdenv</envar> points to the location of the standard environment being used. (It wasn't specified explicitly as an attribute in <xref linkend='ex-hello-nix' />, but <varname>mkDerivation</varname> adds it automatically.)</para> </callout> <callout arearefs='ex-hello-builder-co-2'> <para>Since Hello needs Perl, we have to make sure that Perl is in the <envar>PATH</envar>. The <envar>perl</envar> environment variable points to the location of the Perl package (since it was passed in as an attribute to the derivation), so <filename><replaceable>$perl</replaceable>/bin</filename> is the directory containing the Perl interpreter.</para> </callout> <callout arearefs='ex-hello-builder-co-3'> <para>Now we have to unpack the sources. The <varname>src</varname> attribute was bound to the result of fetching the Hello source tarball from the network, so the <envar>src</envar> environment variable points to the location in the Nix store to which the tarball was downloaded. After unpacking, we <command>cd</command> to the resulting source directory.</para> <para>The whole build is performed in a temporary directory created in <varname>/tmp</varname>, by the way. This directory is removed after the builder finishes, so there is no need to clean up the sources afterwards. Also, the temporary directory is always newly created, so you don't have to worry about files from previous builds interfering with the current build.</para> </callout> <callout arearefs='ex-hello-builder-co-4'> <para>GNU Hello is a typical Autoconf-based package, so we first have to run its <filename>configure</filename> script. In Nix every package is stored in a separate location in the Nix store, for instance <filename>/nix/store/9a54ba97fb71b65fda531012d0443ce2-hello-2.1.1</filename>. Nix computes this path by cryptographically hashing all attributes of the derivation. The path is passed to the builder through the <envar>out</envar> environment variable. So here we give <filename>configure</filename> the parameter <literal>--prefix=$out</literal> to cause Hello to be installed in the expected location.</para> </callout> <callout arearefs='ex-hello-builder-co-5'> <para>Finally we build Hello (<literal>make</literal>) and install it into the location specified by <envar>out</envar> (<literal>make install</literal>).</para> </callout> </calloutlist> <para>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 <option>-e</option> option, which causes the script to be aborted if any command fails without an error check.</para> </section> <section><title>Composition</title> <example xml:id='ex-hello-composition'><title>Composing GNU Hello (<filename>all-packages.nix</filename>)</title> <programlisting> ... rec { <co xml:id='ex-hello-composition-co-1' /> hello = import ../applications/misc/hello/ex-1 <co xml:id='ex-hello-composition-co-2' /> { <co xml:id='ex-hello-composition-co-3' /> inherit fetchurl stdenv perl; }; perl = import ../development/interpreters/perl { <co xml:id='ex-hello-composition-co-4' /> inherit fetchurl stdenv; }; fetchurl = import ../build-support/fetchurl { inherit stdenv; ... }; stdenv = ...; } </programlisting> </example> <para>The Nix expression in <xref linkend='ex-hello-nix' /> is a function; it is missing some arguments that have to be filled in somewhere. In the Nix Packages collection this is done in the file <filename>pkgs/top-level/all-packages.nix</filename>, where all Nix expressions for packages are imported and called with the appropriate arguments. <xref linkend='ex-hello-composition' /> shows some fragments of <filename>all-packages.nix</filename>.</para> <calloutlist> <callout arearefs='ex-hello-composition-co-1'> <para>This file defines a set of attributes, all of which are concrete derivations (i.e., not functions). In fact, we define a <emphasis>mutually recursive</emphasis> set of attributes. That is, the attributes can refer to each other. This is precisely what we want since we want to <quote>plug</quote> the various packages into each other.</para> </callout> <callout arearefs='ex-hello-composition-co-2'> <para>Here we <emphasis>import</emphasis> the Nix expression for GNU Hello. The import operation just loads and returns the specified Nix expression. In fact, we could just have put the contents of <xref linkend='ex-hello-nix' /> in <filename>all-packages.nix</filename> at this point. That would be completely equivalent, but it would make the file rather bulky.</para> <para>Note that we refer to <filename>../applications/misc/hello/ex-1</filename>, not <filename>../applications/misc/hello/ex-1/default.nix</filename>. When you try to import a directory, Nix automatically appends <filename>/default.nix</filename> to the file name.</para> </callout> <callout arearefs='ex-hello-composition-co-3'> <para>This is where the actual composition takes place. Here we <emphasis>call</emphasis> the function imported from <filename>../applications/misc/hello/ex-1</filename> with a set containing the things that the function expects, namely <varname>fetchurl</varname>, <varname>stdenv</varname>, and <varname>perl</varname>. We use inherit again to use the attributes defined in the surrounding scope (we could also have written <literal>fetchurl = fetchurl;</literal>, etc.).</para> <para>The result of this function call is an actual derivation that can be built by Nix (since when we fill in the arguments of the function, what we get is its body, which is the call to <varname>stdenv.mkDerivation</varname> in <xref linkend='ex-hello-nix' />).</para> <note><para>Nixpkgs has a convenience function <function>callPackage</function> that imports and calls a function, filling in any missing arguments by passing the corresponding attribute from the Nixpkgs set, like this: <programlisting> hello = callPackage ../applications/misc/hello/ex-1 { }; </programlisting> If necessary, you can set or override arguments: <programlisting> hello = callPackage ../applications/misc/hello/ex-1 { stdenv = myStdenv; }; </programlisting> </para></note> </callout> <callout arearefs='ex-hello-composition-co-4'> <para>Likewise, we have to instantiate Perl, <varname>fetchurl</varname>, and the standard environment.</para> </callout> </calloutlist> </section> <section><title>Testing</title> <para>You can now try to build Hello. Of course, you could do <literal>nix-env -f pkgs/top-level/all-packages.nix -i hello</literal>, but you may not want to install a possibly broken package just yet. The best way to test the package is by using the command <command linkend="sec-nix-build">nix-build</command>, which builds a Nix expression and creates a symlink named <filename>result</filename> in the current directory: <screen> $ nix-build pkgs/top-level/all-packages.nix -A hello building path `/nix/store/632d2b22514d...-hello-2.1.1' hello-2.1.1/ hello-2.1.1/intl/ hello-2.1.1/intl/ChangeLog <replaceable>...</replaceable> $ ls -l result lrwxrwxrwx ... 2006-09-29 10:43 result -> /nix/store/632d2b22514d...-hello-2.1.1 $ ./result/bin/hello Hello, world!</screen> The <link linkend='opt-attr'><option>-A</option></link> option selects the <literal>hello</literal> attribute from <filename>all-packages.nix</filename>. This is faster than using the symbolic package name specified by the <literal>name</literal> attribute (which also happens to be <literal>hello</literal>) and is unambiguous (there can be multiple packages with the symbolic name <literal>hello</literal>, but there can be only one attribute in a set named <literal>hello</literal>).</para> <para><command>nix-build</command> registers the <filename>./result</filename> symlink as a garbage collection root, so unless and until you delete the <filename>./result</filename> symlink, the output of the build will be safely kept on your system. You can use <command>nix-build</command>’s <option linkend='opt-out-link'>-o</option> switch to give the symlink another name.</para> <para>Nix has a transactional semantics. Once a build finishes successfully, Nix makes a note of this in its database: it registers that the path denoted by <envar>out</envar> is now <quote>valid</quote>. If you try to build the derivation again, Nix will see that the path is already valid and finish immediately. If a build fails, either because it returns a non-zero exit code, because Nix or the builder are killed, or because the machine crashes, then the output paths will not be registered as valid. If you try to build the derivation again, Nix will remove the output paths if they exist (e.g., because the builder died half-way through <literal>make install</literal>) and try again. Note that there is no <quote>negative caching</quote>: Nix doesn't remember that a build failed, and so a failed build can always be repeated. This is because Nix cannot distinguish between permanent failures (e.g., a compiler error due to a syntax error in the source) and transient failures (e.g., a disk full condition).</para> <para>Nix also performs locking. If you run multiple Nix builds simultaneously, and they try to build the same derivation, the first Nix instance that gets there will perform the build, while the others block (or perform other derivations if available) until the build finishes: <screen> $ nix-build pkgs/top-level/all-packages.nix -A hello waiting for lock on `/nix/store/0h5b7hp8d4hqfrw8igvx97x1xawrjnac-hello-2.1.1x'</screen> So it is always safe to run multiple instances of Nix in parallel (which isn’t the case with, say, <command>make</command>).</para> <para>If you have a system with multiple CPUs, you may want to have Nix build different derivations in parallel (insofar as possible). Just pass the option <link linkend='opt-max-jobs'><option>-j <replaceable>N</replaceable></option></link>, where <replaceable>N</replaceable> is the maximum number of jobs to be run in parallel, or set. Typically this should be the number of CPUs.</para> </section> <section><title>The generic builder</title> <para>Recall from <xref linkend='ex-hello-builder' /> that the builder looked something like this: <programlisting> PATH=$perl/bin:$PATH tar xvfz $src cd hello-* ./configure --prefix=$out make make install</programlisting> The builders for almost all Unix packages look like this — set up some environment variables, unpack the sources, configure, build, and install. For this reason the standard environment provides some Bash functions that automate the build process. A builder using the generic build facilities in shown in <xref linkend='ex-hello-builder2' />.</para> <example xml:id='ex-hello-builder2'><title>Build script using the generic build functions</title> <programlisting> buildInputs="$perl" <co xml:id='ex-hello-builder2-co-1' /> source $stdenv/setup <co xml:id='ex-hello-builder2-co-2' /> genericBuild <co xml:id='ex-hello-builder2-co-3' /></programlisting> </example> <calloutlist> <callout arearefs='ex-hello-builder2-co-1'> <para>The <envar>buildInputs</envar> variable tells <filename>setup</filename> to use the indicated packages as <quote>inputs</quote>. This means that if a package provides a <filename>bin</filename> subdirectory, it's added to <envar>PATH</envar>; if it has a <filename>include</filename> subdirectory, it's added to GCC's header search path; and so on.<footnote><para>How does it work? <filename>setup</filename> tries to source the file <filename><replaceable>pkg</replaceable>/nix-support/setup-hook</filename> of all dependencies. These “setup hooks” can then set up whatever environment variables they want; for instance, the setup hook for Perl sets the <envar>PERL5LIB</envar> environment variable to contain the <filename>lib/site_perl</filename> directories of all inputs.</para></footnote> </para> </callout> <callout arearefs='ex-hello-builder2-co-2'> <para>The function <function>genericBuild</function> is defined in the file <literal>$stdenv/setup</literal>.</para> </callout> <callout arearefs='ex-hello-builder2-co-3'> <para>The final step calls the shell function <function>genericBuild</function>, which performs the steps that were done explicitly in <xref linkend='ex-hello-builder' />. The generic builder is smart enough to figure out whether to unpack the sources using <command>gzip</command>, <command>bzip2</command>, etc. It can be customised in many ways; see <xref linkend='sec-standard-environment' />.</para> </callout> </calloutlist> <para>Discerning readers will note that the <envar>buildInputs</envar> could just as well have been set in the Nix expression, like this: <programlisting> buildInputs = [ perl ];</programlisting> The <varname>perl</varname> attribute can then be removed, and the builder becomes even shorter: <programlisting> source $stdenv/setup genericBuild</programlisting> In fact, <varname>mkDerivation</varname> provides a default builder that looks exactly like that, so it is actually possible to omit the builder for Hello entirely.</para> </section> </section> <section><title>The Nix expression language</title> <para>The Nix expression language is a pure, lazy, functional language. Purity means that operations in the language don't have side-effects (for instance, there is no variable assignment). Laziness means that arguments to functions are evaluated only when they are needed. Functional means that functions are <quote>normal</quote> values that can be passed around and manipulated in interesting ways. The language is not a full-featured, general purpose language. Its main job is to describe packages, compositions of packages, and the variability within packages.</para> <para>This section presents the various features of the language.</para> <section xml:id='ssec-values'><title>Values</title> <simplesect><title>Simple values</title> <para>Nix has the following basic data types: <itemizedlist> <listitem> <para><emphasis>Strings</emphasis> can be written in three ways.</para> <para>The most common way is to enclose the string between double quotes, e.g., <literal>"foo bar"</literal>. Strings can span multiple lines. The special characters <literal>"</literal> and <literal>\</literal> and the character sequence <literal>${</literal> must be escaped by prefixing them with a backslash (<literal>\</literal>). Newlines, carriage returns and tabs can be written as <literal>\n</literal>, <literal>\r</literal> and <literal>\t</literal>, respectively.</para> <para>You can include the result of an expression into a string by enclosing it in <literal>${<replaceable>...</replaceable>}</literal>, a feature known as <emphasis>antiquotation</emphasis>. The enclosed expression must evaluate to something that can be coerced into a string (meaning that it must be a string, a path, or a derivation). For instance, rather than writing <programlisting> "--with-freetype2-library=" + freetype + "/lib"</programlisting> (where <varname>freetype</varname> is a derivation), you can instead write the more natural <programlisting> "--with-freetype2-library=${freetype}/lib"</programlisting> The latter is automatically translated to the former. A more complicated example (from the Nix expression for <link xlink:href='http://www.trolltech.com/products/qt'>Qt</link>): <programlisting> configureFlags = " -system-zlib -system-libpng -system-libjpeg ${if openglSupport then "-dlopen-opengl -L${mesa}/lib -I${mesa}/include -L${libXmu}/lib -I${libXmu}/include" else ""} ${if threadSupport then "-thread" else "-no-thread"} ";</programlisting> Note that Nix expressions and strings can be arbitrarily nested; in this case the outer string contains various antiquotations that themselves contain strings (e.g., <literal>"-thread"</literal>), some of which in turn contain expressions (e.g., <literal>${mesa}</literal>).</para> <para>The second way to write string literals is as an <emphasis>indented string</emphasis>, which is enclosed between pairs of <emphasis>double single-quotes</emphasis>, like so: <programlisting> '' This is the first line. This is the second line. This is the third line. ''</programlisting> This kind of string literal intelligently strips indentation from the start of each line. To be precise, it strips from each line a number of spaces equal to the minimal indentation of the string as a whole (disregarding the indentation of empty lines). For instance, the first and second line are indented two space, while the third line is indented four spaces. Thus, two spaces are stripped from each line, so the resulting string is <programlisting> "This is the first line.\nThis is the second line.\n This is the third line.\n"</programlisting> </para> <para>Note that the whitespace and newline following the opening <literal>''</literal> is ignored if there is no non-whitespace text on the initial line.</para> <para>Antiquotation (<literal>${<replaceable>expr</replaceable>}</literal>) is supported in indented strings.</para> <para>Since <literal>${</literal> and <literal>''</literal> have special meaning in indented strings, you need a way to quote them. <literal>${</literal> can be escaped by prefixing it with <literal>''</literal> (that is, two single quotes), i.e., <literal>''${</literal>. <literal>''</literal> can be escaped by prefixing it with <literal>'</literal>, i.e., <literal>'''</literal>. Finally, linefeed, carriage-return and tab characters can be written as <literal>''\n</literal>, <literal>''\r</literal>, <literal>''\t</literal>.</para> <para>Indented strings are primarily useful in that they allow multi-line string literals to follow the indentation of the enclosing Nix expression, and that less escaping is typically necessary for strings representing languages such as shell scripts and configuration files because <literal>''</literal> is much less common than <literal>"</literal>. Example: <programlisting> stdenv.mkDerivation { <replaceable>...</replaceable> postInstall = '' mkdir $out/bin $out/etc cp foo $out/bin echo "Hello World" > $out/etc/foo.conf ${if enableBar then "cp bar $out/bin" else ""} ''; <replaceable>...</replaceable> } </programlisting> </para> <para>Finally, as a convenience, <emphasis>URIs</emphasis> as defined in appendix B of <link xlink:href='http://www.ietf.org/rfc/rfc2396.txt'>RFC 2396</link> can be written <emphasis>as is</emphasis>, without quotes. For instance, the string <literal>"http://example.org/foo.tar.bz2"</literal> can also be written as <literal>http://example.org/foo.tar.bz2</literal>.</para> </listitem> <listitem><para><emphasis>Integers</emphasis>, e.g., <literal>123</literal>.</para></listitem> <listitem><para><emphasis>Paths</emphasis>, e.g., <filename>/bin/sh</filename> or <filename>./builder.sh</filename>. A path must contain at least one slash to be recognised as such; for instance, <filename>builder.sh</filename> is not a path<footnote><para>It's parsed as an expression that selects the attribute <varname>sh</varname> from the variable <varname>builder</varname>.</para></footnote>. If the file name is relative, i.e., if it does not begin with a slash, it is made absolute at parse time relative to the directory of the Nix expression that contained it. For instance, if a Nix expression in <filename>/foo/bar/bla.nix</filename> refers to <filename>../xyzzy/fnord.nix</filename>, the absolute path is <filename>/foo/xyzzy/fnord.nix</filename>.</para></listitem> <listitem><para><emphasis>Booleans</emphasis> with values <literal>true</literal> and <literal>false</literal>.</para></listitem> <listitem><para>The null value, denoted as <literal>null</literal>.</para></listitem> </itemizedlist> </para> </simplesect> <simplesect><title>Lists</title> <para>Lists are formed by enclosing a whitespace-separated list of values between square brackets. For example, <programlisting> [ 123 ./foo.nix "abc" (f { x = y; }) ]</programlisting> defines a list of four elements, the last being the result of a call to the function <varname>f</varname>. Note that function calls have to be enclosed in parentheses. If they had been omitted, e.g., <programlisting> [ 123 ./foo.nix "abc" f { x = y; } ]</programlisting> the result would be a list of five elements, the fourth one being a function and the fifth being a set.</para> </simplesect> <simplesect><title>Sets</title> <para>Sets are really the core of the language, since ultimately the Nix language is all about creating derivations, which are really just sets of attributes to be passed to build scripts.</para> <para>Sets are just a list of name/value pairs (called <emphasis>attributes</emphasis>) enclosed in curly brackets, where each value is an arbitrary expression terminated by a semicolon. For example: <programlisting> { x = 123; text = "Hello"; y = f { bla = 456; }; }</programlisting> This defines a set with attributes named <varname>x</varname>, <varname>text</varname>, <varname>y</varname>. The order of the attributes is irrelevant. An attribute name may only occur once.</para> <para>Attributes can be selected from a set using the <literal>.</literal> operator. For instance, <programlisting> { a = "Foo"; b = "Bar"; }.a</programlisting> evaluates to <literal>"Foo"</literal>. It is possible to provide a default value in an attribute selection using the <literal>or</literal> keyword. For example, <programlisting> { a = "Foo"; b = "Bar"; }.c or "Xyzzy"</programlisting> will evaluate to <literal>"Xyzzy"</literal> because there is no <varname>c</varname> attribute in the set.</para> <para>You can use arbitrary double-quoted strings as attribute names: <programlisting> { "foo ${bar}" = 123; "nix-1.0" = 456; }."foo ${bar}" </programlisting> This will evaluate to <literal>123</literal> (Assuming <literal>bar</literal> is antiquotable). In the case where an attribute name is just a single antiquotation, the quotes can be dropped: <programlisting> { foo = 123; }.${bar} or 456 </programlisting> This will evaluate to <literal>123</literal> if <literal>bar</literal> evaluates to <literal>"foo"</literal> when coerced to a string and <literal>456</literal> otherwise (again assuming <literal>bar</literal> is antiquotable).</para> </simplesect> </section> <section><title>Language constructs</title> <simplesect><title>Recursive sets</title> <para>Recursive sets are just normal sets, but the attributes can refer to each other. For example, <programlisting> rec { x = y; y = 123; }.x </programlisting> evaluates to <literal>123</literal>. Note that without <literal>rec</literal> the binding <literal>x = y;</literal> would refer to the variable <varname>y</varname> in the surrounding scope, if one exists, and would be invalid if no such variable exists. That is, in a normal (non-recursive) set, attributes are not added to the lexical scope; in a recursive set, they are.</para> <para>Recursive sets of course introduce the danger of infinite recursion. For example, <programlisting> rec { x = y; y = x; }.x</programlisting> does not terminate<footnote><para>Actually, Nix detects infinite recursion in this case and aborts (<quote>infinite recursion encountered</quote>).</para></footnote>.</para> </simplesect> <simplesect><title>Let-expressions</title> <para>A let-expression allows you define local variables for an expression. For instance, <programlisting> let x = "foo"; y = "bar"; in x + y</programlisting> evaluates to <literal>"foobar"</literal>. </para> </simplesect> <simplesect><title>Inheriting attributes</title> <para>When defining a set it is often convenient to copy variables from the surrounding lexical scope (e.g., when you want to propagate attributes). This can be shortened using the <literal>inherit</literal> keyword. For instance, <programlisting> let x = 123; in { inherit x; y = 456; }</programlisting> evaluates to <literal>{ x = 123; y = 456; }</literal>. (Note that this works because <varname>x</varname> is added to the lexical scope by the <literal>let</literal> construct.) It is also possible to inherit attributes from another set. For instance, in this fragment from <filename>all-packages.nix</filename>, <programlisting> graphviz = (import ../tools/graphics/graphviz) { inherit fetchurl stdenv libpng libjpeg expat x11 yacc; inherit (xlibs) libXaw; }; xlibs = { libX11 = ...; libXaw = ...; ... } libpng = ...; libjpg = ...; ...</programlisting> the set used in the function call to the function defined in <filename>../tools/graphics/graphviz</filename> inherits a number of variables from the surrounding scope (<varname>fetchurl</varname> ... <varname>yacc</varname>), but also inherits <varname>libXaw</varname> (the X Athena Widgets) from the <varname>xlibs</varname> (X11 client-side libraries) set.</para> </simplesect> <simplesect xml:id="ss-functions"><title>Functions</title> <para>Functions have the following form: <programlisting> <replaceable>pattern</replaceable>: <replaceable>body</replaceable></programlisting> The pattern specifies what the argument of the function must look like, and binds variables in the body to (parts of) the argument. There are three kinds of patterns:</para> <itemizedlist> <listitem><para>If a pattern is a single identifier, then the function matches any argument. Example: <programlisting> let negate = x: !x; concat = x: y: x + y; in if negate true then concat "foo" "bar" else ""</programlisting> Note that <function>concat</function> is a function that takes one argument and returns a function that takes another argument. This allows partial parameterisation (i.e., only filling some of the arguments of a function); e.g., <programlisting> map (concat "foo") [ "bar" "bla" "abc" ]</programlisting> evaluates to <literal>[ "foobar" "foobla" "fooabc" ]</literal>.</para></listitem> <listitem><para>A <emphasis>set pattern</emphasis> of the form <literal>{ name1, name2, …, nameN }</literal> matches a set containing the listed attributes, and binds the values of those attributes to variables in the function body. For example, the function <programlisting> { x, y, z }: z + y + x</programlisting> can only be called with a set containing exactly the attributes <varname>x</varname>, <varname>y</varname> and <varname>z</varname>. No other attributes are allowed. If you want to allow additional arguments, you can use an ellipsis (<literal>...</literal>): <programlisting> { x, y, z, ... }: z + y + x</programlisting> This works on any set that contains at least the three named attributes.</para> <para>It is possible to provide <emphasis>default values</emphasis> for attributes, in which case they are allowed to be missing. A default value is specified by writing <literal><replaceable>name</replaceable> ? <replaceable>e</replaceable></literal>, where <replaceable>e</replaceable> is an arbitrary expression. For example, <programlisting> { x, y ? "foo", z ? "bar" }: z + y + x</programlisting> specifies a function that only requires an attribute named <varname>x</varname>, but optionally accepts <varname>y</varname> and <varname>z</varname>.</para></listitem> <listitem><para>An <literal>@</literal>-pattern requires that the argument matches with the patterns on the left- and right-hand side of the <literal>@</literal>-sign. For example: <programlisting> args@{ x, y, z, ... }: z + y + x + args.a</programlisting> Here <varname>args</varname> is bound to the entire argument, which is further matches against the pattern <literal>{ x, y, z, ... }</literal>.</para></listitem> </itemizedlist> <para>Note that functions do not have names. If you want to give them a name, you can bind them to an attribute, e.g., <programlisting> let concat = { x, y }: x + y; in concat { x = "foo"; y = "bar"; }</programlisting> </para> </simplesect> <simplesect><title>Conditionals</title> <para>Conditionals look like this: <programlisting> if <replaceable>e1</replaceable> then <replaceable>e2</replaceable> else <replaceable>e3</replaceable></programlisting> where <replaceable>e1</replaceable> is an expression that should evaluate to a Boolean value (<literal>true</literal> or <literal>false</literal>).</para> </simplesect> <simplesect><title>Assertions</title> <para>Assertions are generally used to check that certain requirements on or between features and dependencies hold. They look like this: <programlisting> assert <replaceable>e1</replaceable>; <replaceable>e2</replaceable></programlisting> where <replaceable>e1</replaceable> is an expression that should evaluate to a Boolean value. If it evaluates to <literal>true</literal>, <replaceable>e2</replaceable> is returned; otherwise expression evaluation is aborted and a backtrace is printed.</para> <example xml:id='ex-subversion-nix'><title>Nix expression for Subversion</title> <programlisting> { localServer ? false , httpServer ? false , sslSupport ? false , pythonBindings ? false , javaSwigBindings ? false , javahlBindings ? false , stdenv, fetchurl , openssl ? null, httpd ? null, db4 ? null, expat, swig ? null, j2sdk ? null }: assert localServer -> db4 != null; <co xml:id='ex-subversion-nix-co-1' /> assert httpServer -> httpd != null && httpd.expat == expat; <co xml:id='ex-subversion-nix-co-2' /> assert sslSupport -> openssl != null && (httpServer -> httpd.openssl == openssl); <co xml:id='ex-subversion-nix-co-3' /> assert pythonBindings -> swig != null && swig.pythonSupport; assert javaSwigBindings -> swig != null && swig.javaSupport; assert javahlBindings -> j2sdk != null; stdenv.mkDerivation { name = "subversion-1.1.1"; ... openssl = if sslSupport then openssl else null; <co xml:id='ex-subversion-nix-co-4' /> ... }</programlisting> </example> <para><xref linkend='ex-subversion-nix' /> show how assertions are used in the Nix expression for Subversion.</para> <calloutlist> <callout arearefs='ex-subversion-nix-co-1'> <para>This assertion states that if Subversion is to have support for local repositories, then Berkeley DB is needed. So if the Subversion function is called with the <varname>localServer</varname> argument set to <literal>true</literal> but the <varname>db4</varname> argument set to <literal>null</literal>, then the evaluation fails.</para> </callout> <callout arearefs='ex-subversion-nix-co-2'> <para>This is a more subtle condition: if Subversion is built with Apache (<literal>httpServer</literal>) support, then the Expat library (an XML library) used by Subversion should be same as the one used by Apache. This is because in this configuration Subversion code ends up being linked with Apache code, and if the Expat libraries do not match, a build- or runtime link error or incompatibility might occur.</para> </callout> <callout arearefs='ex-subversion-nix-co-3'> <para>This assertion says that in order for Subversion to have SSL support (so that it can access <literal>https</literal> URLs), an OpenSSL library must be passed. Additionally, it says that <emphasis>if</emphasis> Apache support is enabled, then Apache's OpenSSL should match Subversion's. (Note that if Apache support is not enabled, we don't care about Apache's OpenSSL.)</para> </callout> <callout arearefs='ex-subversion-nix-co-4'> <para>The conditional here is not really related to assertions, but is worth pointing out: it ensures that if SSL support is disabled, then the Subversion derivation is not dependent on OpenSSL, even if a non-<literal>null</literal> value was passed. This prevents an unnecessary rebuild of Subversion if OpenSSL changes.</para> </callout> </calloutlist> </simplesect> <simplesect><title>With-expressions</title> <para>A <emphasis>with-expression</emphasis>, <programlisting> with <replaceable>e1</replaceable>; <replaceable>e2</replaceable></programlisting> introduces the set <replaceable>e1</replaceable> into the lexical scope of the expression <replaceable>e2</replaceable>. For instance, <programlisting> let as = { x = "foo"; y = "bar"; }; in with as; x + y</programlisting> evaluates to <literal>"foobar"</literal> since the <literal>with</literal> adds the <varname>x</varname> and <varname>y</varname> attributes of <varname>as</varname> to the lexical scope in the expression <literal>x + y</literal>. The most common use of <literal>with</literal> is in conjunction with the <function>import</function> function. E.g., <programlisting> with (import ./definitions.nix); ...</programlisting> makes all attributes defined in the file <filename>definitions.nix</filename> available as if they were defined locally in a <literal>rec</literal>-expression.</para> </simplesect> <simplesect><title>Comments</title> <para>Comments can be single-line, started with a <literal>#</literal> character, or inline/multi-line, enclosed within <literal>/* ... */</literal>.</para> </simplesect> </section> <section><title>Operators</title> <para><xref linkend='table-operators' /> lists the operators in the Nix expression language, in order of precedence (from strongest to weakest binding).</para> <table xml:id='table-operators'> <title>Operators</title> <tgroup cols='3'> <thead> <row> <entry>Syntax</entry> <entry>Associativity</entry> <entry>Description</entry> </row> </thead> <tbody> <row> <entry><replaceable>e</replaceable> <literal>.</literal> <replaceable>attrpath</replaceable> [ <literal>or</literal> <replaceable>def</replaceable> ] </entry> <entry>none</entry> <entry>Select attribute denoted by the attribute path <replaceable>attrpath</replaceable> from set <replaceable>e</replaceable>. (An attribute path is a dot-separated list of attribute names.) If the attribute doesn’t exist, return <replaceable>def</replaceable> if provided, otherwise abort evaluation.</entry> </row> <row> <entry><replaceable>e1</replaceable> <replaceable>e2</replaceable></entry> <entry>left</entry> <entry>Call function <replaceable>e1</replaceable> with argument <replaceable>e2</replaceable>.</entry> </row> <row> <entry><replaceable>e</replaceable> <literal>?</literal> <replaceable>attrpath</replaceable></entry> <entry>none</entry> <entry>Test whether set <replaceable>e</replaceable> contains the attribute denoted by <replaceable>attrpath</replaceable>; return <literal>true</literal> or <literal>false</literal>.</entry> </row> <row> <entry><replaceable>e1</replaceable> <literal>++</literal> <replaceable>e2</replaceable></entry> <entry>right</entry> <entry>List concatenation.</entry> </row> <row> <entry><replaceable>e1</replaceable> <literal>+</literal> <replaceable>e2</replaceable></entry> <entry>left</entry> <entry>String or path concatenation.</entry> </row> <row> <entry><literal>!</literal> <replaceable>e</replaceable></entry> <entry>left</entry> <entry>Boolean negation.</entry> </row> <row> <entry><replaceable>e1</replaceable> <literal>//</literal> <replaceable>e2</replaceable></entry> <entry>right</entry> <entry>Return a set consisting of the attributes in <replaceable>e1</replaceable> and <replaceable>e2</replaceable> (with the latter taking precedence over the former in case of equally named attributes).</entry> </row> <row> <entry><replaceable>e1</replaceable> <literal>==</literal> <replaceable>e2</replaceable></entry> <entry>none</entry> <entry>Equality.</entry> </row> <row> <entry><replaceable>e1</replaceable> <literal>!=</literal> <replaceable>e2</replaceable></entry> <entry>none</entry> <entry>Inequality.</entry> </row> <row> <entry><replaceable>e1</replaceable> <literal>&&</literal> <replaceable>e2</replaceable></entry> <entry>left</entry> <entry>Logical AND.</entry> </row> <row> <entry><replaceable>e1</replaceable> <literal>||</literal> <replaceable>e2</replaceable></entry> <entry>left</entry> <entry>Logical OR.</entry> </row> <row> <entry><replaceable>e1</replaceable> <literal>-></literal> <replaceable>e2</replaceable></entry> <entry>none</entry> <entry>Logical implication (equivalent to <literal>!<replaceable>e1</replaceable> || <replaceable>e2</replaceable></literal>).</entry> </row> </tbody> </tgroup> </table> </section> <section xml:id="ssec-derivation"><title>Derivations</title> <para>The most important built-in function is <function>derivation</function>, which is used to describe a single derivation (a build action). It takes as input a set, the attributes of which specify the inputs of the build.</para> <itemizedlist> <listitem xml:id="attr-system"><para>There must be an attribute named <varname>system</varname> whose value must be a string specifying a Nix platform identifier, such as <literal>"i686-linux"</literal> or <literal>"powerpc-darwin"</literal><footnote><para>To figure out your platform identifier, look at the line <quote>Checking for the canonical Nix system name</quote> in the output of Nix's <filename>configure</filename> script.</para></footnote> The build can only be performed on a machine and operating system matching the platform identifier. (Nix can automatically forward builds for other platforms by forwarding them to other machines; see <xref linkend='chap-distributed-builds' />.)</para></listitem> <listitem><para>There must be an attribute named <varname>name</varname> whose value must be a string. This is used as a symbolic name for the package by <command>nix-env</command>, and it is appended to the output paths of the derivation.</para></listitem> <listitem><para>There must be an attribute named <varname>builder</varname> that identifies the program that is executed to perform the build. It can be either a derivation or a source (a local file reference, e.g., <filename>./builder.sh</filename>).</para></listitem> <listitem><para>Every attribute is passed as an environment variable to the builder. Attribute values are translated to environment variables as follows: <itemizedlist> <listitem><para>Strings and integers are just passed verbatim.</para></listitem> <listitem><para>A <emphasis>path</emphasis> (e.g., <filename>../foo/sources.tar</filename>) causes the referenced file to be copied to the store; its location in the store is put in the environment variable. The idea is that all sources should reside in the Nix store, since all inputs to a derivation should reside in the Nix store.</para></listitem> <listitem><para>A <emphasis>derivation</emphasis> causes that derivation to be built prior to the present derivation; its default output path is put in the environment variable.</para></listitem> <listitem><para>Lists of the previous types are also allowed. They are simply concatenated, separated by spaces.</para></listitem> <listitem><para><literal>true</literal> is passed as the string <literal>1</literal>, <literal>false</literal> and <literal>null</literal> are passed as an empty string. </para></listitem> </itemizedlist> </para></listitem> <listitem><para>The optional attribute <varname>args</varname> specifies command-line arguments to be passed to the builder. It should be a list.</para></listitem> <listitem><para>The optional attribute <varname>outputs</varname> specifies a list of symbolic outputs of the derivation. By default, a derivation produces a single output path, denoted as <literal>out</literal>. However, derivations can produce multiple output paths. This is useful because it allows outputs to be downloaded or garbage-collected separately. For instance, imagine a library package that provides a dynamic library, header files, and documentation. A program that links against the library doesn’t need the header files and documentation at runtime, and it doesn’t need the documentation at build time. Thus, the library package could specify: <programlisting> outputs = [ "lib" "headers" "doc" ]; </programlisting> This will cause Nix to pass environment variables <literal>lib</literal>, <literal>headers</literal> and <literal>doc</literal> to the builder containing the intended store paths of each output. The builder would typically do something like <programlisting> ./configure --libdir=$lib/lib --includedir=$headers/include --docdir=$doc/share/doc </programlisting> for an Autoconf-style package. You can refer to each output of a derivation by selecting it as an attribute, e.g. <programlisting> buildInputs = [ pkg.lib pkg.headers ]; </programlisting> The first element of <varname>output</varname> determines the <emphasis>default output</emphasis>. Thus, you could also write <programlisting> buildInputs = [ pkg pkg.headers ]; </programlisting> since <literal>pkg</literal> is equivalent to <literal>pkg.lib</literal>.</para></listitem> </itemizedlist> <para>The function <function>mkDerivation</function> in the standard environment is a wrapper around <function>derivation</function> that adds a default value for <varname>system</varname> and always uses Bash as the builder, to which the supplied builder is passed as a command-line argument. See <xref linkend='sec-standard-environment' />.</para> <para>The builder is executed as follows: <itemizedlist> <listitem><para>A temporary directory is created under the directory specified by <envar>TMPDIR</envar> (default <filename>/tmp</filename>) where the build will take place. The current directory is changed to this directory.</para></listitem> <listitem><para>The environment is cleared and set to the derivation attributes, as specified above.</para></listitem> <listitem><para>In addition, the following variables are set: <itemizedlist> <listitem><para><envar>NIX_BUILD_TOP</envar> contains the path of the temporary directory for this build.</para></listitem> <listitem><para>Also, <envar>TMPDIR</envar>, <envar>TEMPDIR</envar>, <envar>TMP</envar>, <envar>TEMP</envar> are set to point to the temporary directory. This is to prevent the builder from accidentally writing temporary files anywhere else. Doing so might cause interference by other processes.</para></listitem> <listitem><para><envar>PATH</envar> is set to <filename>/path-not-set</filename> to prevent shells from initialising it to their built-in default value.</para></listitem> <listitem><para><envar>HOME</envar> is set to <filename>/homeless-shelter</filename> to prevent programs from using <filename>/etc/passwd</filename> or the like to find the user's home directory, which could cause impurity. Usually, when <envar>HOME</envar> is set, it is used as the location of the home directory, even if it points to a non-existent path.</para></listitem> <listitem><para><envar>NIX_STORE</envar> is set to the path of the top-level Nix store directory (typically, <filename>/nix/store</filename>).</para></listitem> <listitem><para>For each output declared in <varname>outputs</varname>, the corresponding environment variable is set to point to the intended path in the Nix store for that output. Each output path is a concatenation of the cryptographic hash of all build inputs, the <varname>name</varname> attribute and the output name. (The output name is omitted if it’s <literal>out</literal>.)</para></listitem> </itemizedlist> </para></listitem> <listitem><para>If an output path already exists, it is removed. Also, locks are acquired to prevent multiple Nix instances from performing the same build at the same time.</para></listitem> <listitem><para>A log of the combined standard output and error is written to <filename>/nix/var/log/nix</filename>.</para></listitem> <listitem><para>The builder is executed with the arguments specified by the attribute <varname>args</varname>. If it exits with exit code 0, it is considered to have succeeded.</para></listitem> <listitem><para>The temporary directory is removed (unless the <option>-K</option> option was specified).</para></listitem> <listitem><para>If the build was successful, Nix scans each output path for references to input paths by looking for the hash parts of the input paths. Since these are potential runtime dependencies, Nix registers them as dependencies of the output paths.</para></listitem> <listitem><para>After the build, Nix sets the last-modified timestamp on all files in the build result to 1 (00:00:01 1/1/1970 UTC), sets the group to the default group, and sets the mode of the file to 0444 or 0555 (i.e., read-only, with execute permission enabled if the file was originally executable). Note that possible <literal>setuid</literal> and <literal>setgid</literal> bits are cleared. Setuid and setgid programs are not currently supported by Nix. This is because the Nix archives used in deployment have no concept of ownership information, and because it makes the build result dependent on the user performing the build.</para></listitem> </itemizedlist> </para> <section><title>Advanced attributes</title> <para>Derivations can declare some infrequently used optional attributes.</para> <variablelist> <varlistentry><term><varname>allowedReferences</varname></term> <listitem><para>The optional attribute <varname>allowedReferences</varname> specifies a list of legal references (dependencies) of the output of the builder. For example, <programlisting> allowedReferences = []; </programlisting> enforces that the output of a derivation cannot have any runtime dependencies on its inputs. This is used in NixOS to check that generated files such as initial ramdisks for booting Linux don’t have accidental dependencies on other paths in the Nix store.</para></listitem> </varlistentry> <varlistentry><term><varname>exportReferencesGraph</varname></term> <listitem><para>This attribute allows builders access to the references graph of their inputs. The attribute is a list of inputs in the Nix store whose references graph the builder needs to know. The value of this attribute should be a list of pairs <literal>[ <replaceable>name1</replaceable> <replaceable>path1</replaceable> <replaceable>name2</replaceable> <replaceable>path2</replaceable> <replaceable>...</replaceable> ]</literal>. The references graph of each <replaceable>pathN</replaceable> will be stored in a text file <replaceable>nameN</replaceable> in the temporary build directory. The text files have the format used by <command>nix-store --register-validity</command> (with the deriver fields left empty). For example, when the following derivation is built: <programlisting> derivation { ... exportReferencesGraph = [ "libfoo-graph" libfoo ]; }; </programlisting> the references graph of <literal>libfoo</literal> is placed in the file <filename>libfoo-graph</filename> in the temporary build directory.</para> <para><varname>exportReferencesGraph</varname> is useful for builders that want to do something with the closure of a store path. Examples include the builders in NixOS that generate the initial ramdisk for booting Linux (a <command>cpio</command> archive containing the closure of the boot script) and the ISO-9660 image for the installation CD (which is populated with a Nix store containing the closure of a bootable NixOS configuration).</para></listitem> </varlistentry> <varlistentry xml:id="fixed-output-drvs"> <term><varname>outputHash</varname></term> <term><varname>outputHashAlgo</varname></term> <term><varname>outputHashMode</varname></term> <listitem><para>These attributes declare that the derivation is a so-called <emphasis>fixed-output derivation</emphasis>, which means that a cryptographic hash of the output is already known in advance. When the build of a fixed-output derivation finishes, Nix computes the cryptographic hash of the output and compares it to the hash declared with these attributes. If there is a mismatch, the build fails.</para> <para>The rationale for fixed-output derivations is derivations such as those produced by the <function>fetchurl</function> function. This function downloads a file from a given URL. To ensure that the downloaded file has not been modified, the caller must also specify a cryptographic hash of the file. For example, <programlisting> fetchurl { url = http://ftp.gnu.org/pub/gnu/hello/hello-2.1.1.tar.gz; md5 = "70c9ccf9fac07f762c24f2df2290784d"; } </programlisting> It sometimes happens that the URL of the file changes, e.g., because servers are reorganised or no longer available. We then must update the call to <function>fetchurl</function>, e.g., <programlisting> fetchurl { url = ftp://ftp.nluug.nl/pub/gnu/hello/hello-2.1.1.tar.gz; md5 = "70c9ccf9fac07f762c24f2df2290784d"; } </programlisting> If a <function>fetchurl</function> derivation was treated like a normal derivation, the output paths of the derivation and <emphasis>all derivations depending on it</emphasis> would change. For instance, if we were to change the URL of the Glibc source distribution in Nixpkgs (a package on which almost all other packages depend) massive rebuilds would be needed. This is unfortunate for a change which we know cannot have a real effect as it propagates upwards through the dependency graph.</para> <para>For fixed-output derivations, on the other hand, the name of the output path only depends on the <varname>outputHash*</varname> and <varname>name</varname> attributes, while all other attributes are ignored for the purpose of computing the output path. (The <varname>name</varname> attribute is included because it is part of the path.)</para> <para>As an example, here is the (simplified) Nix expression for <varname>fetchurl</varname>: <programlisting> { stdenv, curl }: # The <command>curl</command> program is used for downloading. { url, md5 }: stdenv.mkDerivation { name = baseNameOf (toString url); builder = ./builder.sh; buildInputs = [ curl ]; # This is a fixed-output derivation; the output must be a regular # file with MD5 hash <varname>md5</varname>. outputHashMode = "flat"; outputHashAlgo = "md5"; outputHash = md5; inherit url; } </programlisting> </para> <para>The <varname>outputHashAlgo</varname> attribute specifies the hash algorithm used to compute the hash. It can currently be <literal>"md5"</literal>, <literal>"sha1"</literal> or <literal>"sha256"</literal>.</para> <para>The <varname>outputHashMode</varname> attribute determines how the hash is computed. It must be one of the following two values: <variablelist> <varlistentry><term><literal>"flat"</literal></term> <listitem><para>The output must be a non-executable regular file. If it isn’t, the build fails. The hash is simply computed over the contents of that file (so it’s equal to what Unix commands like <command>md5sum</command> or <command>sha1sum</command> produce).</para> <para>This is the default.</para></listitem> </varlistentry> <varlistentry><term><literal>"recursive"</literal></term> <listitem><para>The hash is computed over the NAR archive dump of the output (i.e., the result of <link linkend="refsec-nix-store-dump"><command>nix-store --dump</command></link>). In this case, the output can be anything, including a directory tree.</para></listitem> </varlistentry> </variablelist> </para> <para>The <varname>outputHash</varname> attribute, finally, must be a string containing the hash in either hexadecimal or base-32 notation. (See the <link linkend="sec-nix-hash"><command>nix-hash</command> command</link> for information about converting to and from base-32 notation.)</para></listitem> </varlistentry> <varlistentry><term><varname>impureEnvVars</varname></term> <listitem><para>This attribute allows you to specify a list of environment variables that should be passed from the environment of the calling user to the builder. Usually, the environment is cleared completely when the builder is executed, but with this attribute you can allow specific environment variables to be passed unmodified. For example, <function>fetchurl</function> in Nixpkgs has the line <programlisting> impureEnvVars = [ "http_proxy" "https_proxy" <replaceable>...</replaceable> ]; </programlisting> to make it use the proxy server configuration specified by the user in the environment variables <envar>http_proxy</envar> and friends.</para> <para>This attribute is only allowed in <link linkend="fixed-output-drvs">fixed-output derivations</link>, where impurities such as these are okay since (the hash of) the output is known in advance. It is ignored for all other derivations.</para></listitem> </varlistentry> <varlistentry><term><varname>preferLocalBuild</varname></term> <listitem><para>If this attribute is set to <literal>true</literal>, it has two effects. First, the derivation will always be built, not substituted, even if a substitute is available. Second, if <link linkend="chap-distributed-builds">distributed building is enabled</link>, then, if possible, the derivaton will be built locally instead of forwarded to a remote machine. This is appropriate for trivial builders where the cost of doing a download or remote build would exceed the cost of building locally.</para></listitem> </varlistentry> </variablelist> </section> </section> <xi:include href="builtins.xml" /> </section> <section xml:id='sec-standard-environment'><title>The standard environment</title> <para>The standard environment is used by passing it as an input called <envar>stdenv</envar> to the derivation, and then doing <programlisting> source $stdenv/setup</programlisting> at the top of the builder.</para> <para>Apart from adding the aforementioned commands to the <envar>PATH</envar>, <filename>setup</filename> also does the following: <itemizedlist> <listitem><para>All input packages specified in the <envar>buildInputs</envar> environment variable have their <filename>/bin</filename> subdirectory added to <envar>PATH</envar>, their <filename>/include</filename> subdirectory added to the C/C++ header file search path, and their <filename>/lib</filename> subdirectory added to the linker search path. This can be extended. For instance, when the <command>pkgconfig</command> package is used, the subdirectory <filename>/lib/pkgconfig</filename> of each input is added to the <envar>PKG_CONFIG_PATH</envar> environment variable.</para></listitem> <listitem><para>The environment variable <envar>NIX_CFLAGS_STRIP</envar> is set so that the compiler strips debug information from object files. This can be disabled by setting <envar>NIX_STRIP_DEBUG</envar> to <literal>0</literal>.</para></listitem> </itemizedlist> </para> <para>The <filename>setup</filename> script also exports a function called <function>genericBuild</function> that knows how to build typical Autoconf-style packages. It can be customised to perform builds for any type of package. It is advisable to use <function>genericBuild</function> since it provides facilities that are almost always useful such as unpacking of sources, patching of sources, nested logging, etc.</para> <para>The definitive, up-to-date documentation of the generic builder is the source itself, which resides in <filename>pkgs/stdenv/generic/setup.sh</filename>.</para> <section><title>Customising the generic builder</title> <para>The operation of the generic builder can be modified in many places by setting certain variables. These <emphasis>hook variables</emphasis> are typically set to the name of some shell function defined by you. For instance, to perform some additional steps after <command>make install</command> you would set the <varname>postInstall</varname> variable: <programlisting> postInstall=myPostInstall myPostInstall() { mkdir $out/share/extra cp extrafiles/* $out/share/extra }</programlisting> </para> </section> <section><title>Debugging failed builds</title> <para>At the beginning of each phase, the set of all shell variables is written to the file <filename>env-vars</filename> at the top-level build directory. This is useful for debugging: it allows you to recreate the environment in which a build was performed. For instance, if a build fails, then assuming you used the <option>-K</option> flag, you can go to the output directory and <quote>switch</quote> to the environment of the builder: <screen> $ nix-build -K ./foo.nix ... fails, keeping build directory `/tmp/nix-1234-0' $ cd /tmp/nix-1234-0 $ source env-vars <lineannotation>(edit some files...)</lineannotation> $ make <lineannotation>(execution continues with the same GCC, make, etc.)</lineannotation></screen> </para> </section> </section> </chapter>