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Merged to R9561; Fixed initial snapshot bug.

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Wouter den Breejen 2007-10-31 15:08:22 +00:00
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2
doc/manual/Makefile.am
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@ -20,7 +20,7 @@ man1_MANS = nix-env.1 nix-build.1 nix-store.1 nix-instantiate.1 \
FIGURES = figures/user-environments.png
MANUAL_SRCS = manual.xml introduction.xml installation.xml \
package-management.xml writing-nix-expressions.xml \
package-management.xml writing-nix-expressions.xml builtins.xml \
build-farm.xml \
$(man1_MANS:.1=.xml) \
troubleshooting.xml bugs.xml opt-common.xml opt-common-syn.xml \

751
doc/manual/builtins.xml Normal file
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@ -0,0 +1,751 @@
<section xmlns="http://docbook.org/ns/docbook"
xmlns:xlink="http://www.w3.org/1999/xlink"
xml:id='ssec-builtins'>
<title>Built-in functions</title>
<para>This section lists the functions and constants built into the
Nix expression evaluator. (The built-in function
<function>derivation</function> is discussed above.) Some built-ins,
such as <function>derivation</function>, are always in scope of every
Nix expression; you can just access them right away. But to prevent
polluting the namespace too much, most built-ins are not in scope.
Instead, you can access them through the <varname>builtins</varname>
built-in value, which is an attribute set that contains all built-in
functions and values. For instance, <function>derivation</function>
is also available as <function>builtins.derivation</function>.</para>
<variablelist >
<varlistentry><term><function>abort</function> <replaceable>s</replaceable></term>
<listitem><para>Abort Nix expression evaluation, print error
message <replaceable>s</replaceable>.</para></listitem>
</varlistentry>
<varlistentry><term><function>builtins.add</function>
<replaceable>e1</replaceable> <replaceable>e2</replaceable></term>
<listitem><para>Return the sum of the integers
<replaceable>e1</replaceable> and
<replaceable>e2</replaceable>.</para></listitem>
</varlistentry>
<varlistentry><term><function>builtins.attrNames</function>
<replaceable>attrs</replaceable></term>
<listitem><para>Return the names of the attributes in the
attribute set <replaceable>attrs</replaceable> in a sorted list.
For instance, <literal>builtins.attrNames {y = 1; x =
"foo";}</literal> evaluates to <literal>["x" "y"]</literal>.
There is no built-in function <function>attrValues</function>, but
you can easily define it yourself:
<programlisting>
attrValues = attrs: map (name: builtins.getAttr name attrs) (builtins.attrNames attrs);</programlisting>
</para></listitem>
</varlistentry>
<varlistentry><term><function>baseNameOf</function> <replaceable>s</replaceable></term>
<listitem><para>Return the <emphasis>base name</emphasis> of the
string <replaceable>s</replaceable>, that is, everything following
the final slash in the string. This is similar to the GNU
<command>basename</command> command.</para></listitem>
</varlistentry>
<varlistentry><term><varname>builtins</varname></term>
<listitem><para>The attribute set <varname>builtins</varname>
contains all the built-in functions and values. You can use
<varname>builtins</varname> to test for the availability of
features in the Nix installation, e.g.,
<programlisting>
if builtins ? getEnv then builtins.getEnv "PATH" else ""</programlisting>
This allows a Nix expression to fall back gracefully on older Nix
installations that don’t have the desired built-in function.
However, in that case you should not write
<programlisting>
if builtins ? getEnv then __getEnv "PATH" else ""</programlisting>
This Nix expression will trigger an “undefined variable” error on
older Nix versions since <function>__getEnv</function> doesn’t
exist. <literal>builtins.getEnv</literal>, on the other hand, is
safe since <literal>builtins</literal> always exists and attribute
selection is lazy, so it’s only performed if the test
succeeds.</para></listitem>
</varlistentry>
<varlistentry
xml:id='builtin-currentSystem'><term><varname>builtins.currentSystem</varname></term>
<listitem><para>The built-in value <varname>currentSystem</varname>
evaluates to the Nix platform identifier for the Nix installation
on which the expression is being evaluated, such as
<literal>"i686-linux"</literal> or
<literal>"powerpc-darwin"</literal>.</para></listitem>
</varlistentry>
<!--
<varlistentry><term><function>currentTime</function></term>
<listitem><para>The built-in value <varname>currentTime</varname>
returns the current system time in seconds since 00:00:00 1/1/1970
UTC. Due to the evaluation model of Nix expressions
(<emphasis>maximal laziness</emphasis>), it always yields the same
value within an execution of Nix.</para></listitem>
</varlistentry>
-->
<!--
<varlistentry><term><function>dependencyClosure</function></term>
<listitem><para>TODO</para></listitem>
</varlistentry>
-->
<varlistentry><term><function>derivation</function>
<replaceable>attrs</replaceable></term>
<listitem><para><function>derivation</function> is described in
<xref linkend='ssec-derivation' />.</para></listitem>
</varlistentry>
<varlistentry><term><function>dirOf</function> <replaceable>s</replaceable></term>
<listitem><para>Return the directory part of the string
<replaceable>s</replaceable>, that is, everything before the final
slash in the string. This is similar to the GNU
<command>dirname</command> command.</para></listitem>
</varlistentry>
<varlistentry><term><function>builtins.filterSource</function>
<replaceable>e1</replaceable> <replaceable>e2</replaceable></term>
<listitem>
<para>This function allows you to copy sources into the Nix
store while filtering certain files. For instance, suppose that
you want to use the directory <filename>source-dir</filename> as
an input to a Nix expression, e.g.
<programlisting>
stdenv.mkDerivation {
...
src = ./source-dir;
}
</programlisting>
However, if <filename>source-dir</filename> is a Subversion
working copy, then all those annoying <filename>.svn</filename>
subdirectories will also be copied to the store. Worse, the
contents of those directories may change a lot, causing lots of
spurious rebuilds. With <function>filterSource</function> you
can filter out the <filename>.svn</filename> directories:
<programlisting>
src = builtins.filterSource
(path: type: type != "directory" || baseNameOf path != ".svn")
./source-dir;
</programlisting>
</para>
<para>Thus, the first argument <replaceable>e1</replaceable>
must be a predicate function that is called for each regular
file, directory or symlink in the source tree
<replaceable>e2</replaceable>. If the function returns
<literal>true</literal>, the file is copied to the Nix store,
otherwise it is omitted. The function is called with two
arguments. The first is the full path of the file. The second
is a string that identifies the type of the file, which is
either <literal>"regular"</literal>,
<literal>"directory"</literal>, <literal>"symlink"</literal> or
<literal>"unknown"</literal> (for other kinds of files such as
device nodes or fifos — but note that those cannot be copied to
the Nix store, so if the predicate returns
<literal>true</literal> for them, the copy will fail).</para>
</listitem>
</varlistentry>
<varlistentry><term><function>builtins.getAttr</function>
<replaceable>s</replaceable> <replaceable>attrs</replaceable></term>
<listitem><para><function>getAttr</function> returns the attribute
named <replaceable>s</replaceable> from the attribute set
<replaceable>attrs</replaceable>. Evaluation aborts if the
attribute doesn’t exist. This is a dynamic version of the
<literal>.</literal> operator, since <replaceable>s</replaceable>
is an expression rather than an identifier.</para></listitem>
</varlistentry>
<varlistentry><term><function>builtins.getEnv</function>
<replaceable>s</replaceable></term>
<listitem><para><function>getEnv</function> returns the value of
the environment variable <replaceable>s</replaceable>, or an empty
string if the variable doesn’t exist. This function should be
used with care, as it can introduce all sorts of nasty environment
dependencies in your Nix expression.</para>
<para><function>getEnv</function> is used in Nix Packages to
locate the file <filename>~/.nixpkgs/config.nix</filename>, which
contains user-local settings for Nix Packages. (That is, it does
a <literal>getEnv "HOME"</literal> to locate the user’s home
directory.)</para></listitem>
</varlistentry>
<varlistentry><term><function>builtins.hasAttr</function>
<replaceable>s</replaceable> <replaceable>attrs</replaceable></term>
<listitem><para><function>hasAttr</function> returns
<literal>true</literal> if the attribute set
<replaceable>attrs</replaceable> has an attribute named
<replaceable>s</replaceable>, and <literal>false</literal>
otherwise. This is a dynamic version of the <literal>?</literal>
operator, since <replaceable>s</replaceable> is an expression
rather than an identifier.</para></listitem>
</varlistentry>
<varlistentry><term><function>builtins.head</function>
<replaceable>list</replaceable></term>
<listitem><para>Return the first element of a list; abort
evaluation if the argument isn’t a list or is an empty list. You
can test whether a list is empty by comparing it with
<literal>[]</literal>.</para></listitem>
</varlistentry>
<varlistentry><term><function>import</function>
<replaceable>path</replaceable></term>
<listitem><para>Load, parse and return the Nix expression in the
file <replaceable>path</replaceable>. Evaluation aborts if the
file doesn’t exist or contains an incorrect Nix
expression. <function>import</function> implements Nix’s module
system: you can put any Nix expression (such as an attribute set
or a function) in a separate file, and use it from Nix expressions
in other files.</para>
<para>A Nix expression loaded by <function>import</function> must
not contain any <emphasis>free variables</emphasis> (identifiers
that are not defined in the Nix expression itself and are not
built-in). Therefore, it cannot refer to variables that are in
scope at the call site. For instance, if you have a calling
expression
<programlisting>
rec {
x = 123;
y = import ./foo.nix;
}</programlisting>
then the following <filename>foo.nix</filename> will give an
error:
<programlisting>
x + 456</programlisting>
since <varname>x</varname> is not in scope in
<filename>foo.nix</filename>. If you want <varname>x</varname>
to be available in <filename>foo.nix</filename>, you should pass
it as a function argument:
<programlisting>
rec {
x = 123;
y = import ./foo.nix x;
}</programlisting>
and
<programlisting>
x: x + 456</programlisting>
(The function argument doesn’t have to be called
<varname>x</varname> in <filename>foo.nix</filename>; any name
would work.)</para></listitem>
</varlistentry>
<varlistentry><term><function>builtins.isAttrs</function>
<replaceable>e</replaceable></term>
<listitem><para>Return <literal>true</literal> if
<replaceable>e</replaceable> evaluates to an attribute set, and
<literal>false</literal> otherwise.</para></listitem>
</varlistentry>
<varlistentry><term><function>builtins.isList</function>
<replaceable>e</replaceable></term>
<listitem><para>Return <literal>true</literal> if
<replaceable>e</replaceable> evaluates to a list, and
<literal>false</literal> otherwise.</para></listitem>
</varlistentry>
<varlistentry><term><function>builtins.isFunction</function>
<replaceable>e</replaceable></term>
<listitem><para>Return <literal>true</literal> if
<replaceable>e</replaceable> evaluates to a function, and
<literal>false</literal> otherwise.</para></listitem>
</varlistentry>
<varlistentry><term><function>isNull</function>
<replaceable>e</replaceable></term>
<listitem><para>Return <literal>true</literal> if
<replaceable>e</replaceable> evaluates to <literal>null</literal>,
and <literal>false</literal> otherwise.</para>
<warning><para>This function is <emphasis>deprecated</emphasis>;
just write <literal>e == null</literal> instead.</para></warning>
</listitem>
</varlistentry>
<varlistentry><term><function>builtins.lessThan</function>
<replaceable>e1</replaceable> <replaceable>e2</replaceable></term>
<listitem><para>Return <literal>true</literal> if the integer
<replaceable>e1</replaceable> is less than the integer
<replaceable>e2</replaceable>, and <literal>false</literal>
otherwise. Evaluation aborts if either
<replaceable>e1</replaceable> or <replaceable>e2</replaceable>
does not evaluate to an integer.</para></listitem>
</varlistentry>
<varlistentry><term><function>builtins.listToAttrs</function>
<replaceable>e</replaceable></term>
<listitem><para>Construct an attribute set from a list specifying
the names and values of each attribute. Each element of the list
should be an attribute set consisting of a string-valued attribute
<varname>name</varname> specifying the name of the attribute, and
an attribute <varname>value</varname> specifying its value.
Example:
<programlisting>
builtins.listToAttrs [
{name = "foo"; value = 123;}
{name = "bar"; value = 456;}
]
</programlisting>
evaluates to
<programlisting>
{ foo = 123; bar = 456; }
</programlisting>
</para></listitem>
</varlistentry>
<varlistentry><term><function>map</function>
<replaceable>f</replaceable> <replaceable>list</replaceable></term>
<listitem><para>Apply the function <replaceable>f</replaceable> to
each element in the list <replaceable>list</replaceable>. For
example,
<programlisting>
map (x: "foo" + x) ["bar" "bla" "abc"]</programlisting>
evaluates to <literal>["foobar" "foobla"
"fooabc"]</literal>.</para></listitem>
</varlistentry>
<varlistentry><term><function>builtins.pathExists</function>
<replaceable>path</replaceable></term>
<listitem><para>Return <literal>true</literal> if the path
<replaceable>path</replaceable> exists, and
<literal>false</literal> otherwise. One application of this
function is to conditionally include a Nix expression containing
user configuration:
<programlisting>
let
fileName = builtins.getEnv "CONFIG_FILE";
config =
if fileName != "" &amp;&amp; builtins.pathExists (builtins.toPath fileName)
then import (builtins.toPath fileName)
else { someSetting = false; }; <lineannotation># default configuration</lineannotation>
in config.someSetting</programlisting>
(Note that <envar>CONFIG_FILE</envar> must be an absolute path for
this to work.)</para></listitem>
</varlistentry>
<!--
<varlistentry><term><function>relativise</function></term>
<listitem><para>TODO</para></listitem>
</varlistentry>
-->
<varlistentry><term><function>removeAttrs</function>
<replaceable>attrs</replaceable> <replaceable>list</replaceable></term>
<listitem><para>Remove the attributes listed in
<replaceable>list</replaceable> from the attribute set
<replaceable>attrs</replaceable>. The attributes don’t have to
exist in <replaceable>attrs</replaceable>. For instance,
<screen>
removeAttrs { x = 1; y = 2; z = 3; } ["a" "x" "z"]</screen>
evaluates to <literal>{y = 2;}</literal>.</para></listitem>
</varlistentry>
<varlistentry><term><function>builtins.stringLength</function>
<replaceable>e</replaceable></term>
<listitem><para>Return the length of the string
<replaceable>e</replaceable>. If <replaceable>e</replaceable> is
not a string, evaluation is aborted.</para></listitem>
</varlistentry>
<varlistentry><term><function>builtins.sub</function>
<replaceable>e1</replaceable> <replaceable>e2</replaceable></term>
<listitem><para>Return the difference between the integers
<replaceable>e1</replaceable> and
<replaceable>e2</replaceable>.</para></listitem>
</varlistentry>
<varlistentry><term><function>builtins.substr</function>
<replaceable>start</replaceable> <replaceable>len</replaceable>
<replaceable>s</replaceable></term>
<listitem><para>Return the substring of
<replaceable>s</replaceable> from character position
<replaceable>start</replaceable> (zero-based) up to but not
including <replaceable>start + len</replaceable>. If
<replaceable>start</replaceable> is greater than the length of the
string, an empty string is returned, and if <replaceable>start +
len</replaceable> lies beyond the end of the string, only the
substring up to the end of the string is returned.
<replaceable>start</replaceable> must be
non-negative.</para></listitem>
</varlistentry>
<varlistentry><term><function>builtins.tail</function>
<replaceable>list</replaceable></term>
<listitem><para>Return the second to last elements of a list;
abort evaluation if the argument isn’t a list or is an empty
list.</para></listitem>
</varlistentry>
<varlistentry><term><function>throw</function>
<replaceable>s</replaceable></term>
<listitem><para>Throw an error message
<replaceable>s</replaceable>. This usually aborts Nix expression
evaluation, but in <command>nix-env -qa</command> and other
commands that try to evaluate a set of derivations to get
information about those derivations, a derivation that throws an
error is silently skipped (which is not the case for
<function>abort</function>).</para></listitem>
</varlistentry>
<varlistentry
xml:id='builtin-toFile'><term><function>builtins.toFile</function>
<replaceable>name</replaceable> <replaceable>s</replaceable></term>
<listitem><para>Store the string <replaceable>s</replaceable> in a
file in the Nix store and return its path. The file has suffix
<replaceable>name</replaceable>. This file can be used as an
input to derivations. One application is to write builders
“inline”. For instance, the following Nix expression combines
<xref linkend='ex-hello-nix' /> and <xref
linkend='ex-hello-builder' /> into one file:
<programlisting>
{stdenv, fetchurl, perl}:
stdenv.mkDerivation {
name = "hello-2.1.1";
builder = builtins.toFile "builder.sh" "
source $stdenv/setup
PATH=$perl/bin:$PATH
tar xvfz $src
cd hello-*
./configure --prefix=$out
make
make install
";
src = fetchurl {
url = http://nix.cs.uu.nl/dist/tarballs/hello-2.1.1.tar.gz;
md5 = "70c9ccf9fac07f762c24f2df2290784d";
};
inherit perl;
}</programlisting>
</para>
<para>It is even possible for one file to refer to another, e.g.,
<programlisting>
builder = let
configFile = builtins.toFile "foo.conf" "
# This is some dummy configuration file.
<replaceable>...</replaceable>
";
in builtins.toFile "builder.sh" "
source $stdenv/setup
<replaceable>...</replaceable>
cp ${configFile} $out/etc/foo.conf
";</programlisting>
Note that <literal>${configFile}</literal> is an antiquotation
(see <xref linkend='ssec-values' />), so the result of the
expression <literal>configFile</literal> (i.e., a path like
<filename>/nix/store/m7p7jfny445k...-foo.conf</filename>) will be
spliced into the resulting string.</para>
<para>It is however <emphasis>not</emphasis> allowed to have files
mutually referring to each other, like so:
<programlisting>
let
foo = builtins.toFile "foo" "...${bar}...";
bar = builtins.toFile "bar" "...${foo}...";
in foo</programlisting>
This is not allowed because it would cause a cyclic dependency in
the computation of the cryptographic hashes for
<varname>foo</varname> and <varname>bar</varname>.</para></listitem>
</varlistentry>
<varlistentry><term><function>builtins.toPath</function> <replaceable>s</replaceable></term>
<listitem><para>Convert the string value
<replaceable>s</replaceable> into a path value. The string
<replaceable>s</replaceable> must represent an absolute path
(i.e., must start with <literal>/</literal>). The path need not
exist. The resulting path is canonicalised, e.g.,
<literal>builtins.toPath "//foo/xyzzy/../bar/"</literal> returns
<literal>/foo/bar</literal>.</para></listitem>
</varlistentry>
<varlistentry><term><function>toString</function> <replaceable>e</replaceable></term>
<listitem><para>Convert the expression
<replaceable>e</replaceable> to a string.
<replaceable>e</replaceable> can be a string (in which case
<function>toString</function> is a no-op) or a path (e.g.,
<literal>toString /foo/bar</literal> yields
<literal>"/foo/bar"</literal>.</para></listitem>
</varlistentry>
<varlistentry xml:id='builtin-toXML'><term><function>builtins.toXML</function> <replaceable>e</replaceable></term>
<listitem><para>Return a string containing an XML representation
of <replaceable>e</replaceable>. The main application for
<function>toXML</function> is to communicate information with the
builder in a more structured format than plain environment
variables.</para>
<!-- TODO: more formally describe the schema of the XML
representation -->
<para><xref linkend='ex-toxml' /> shows an example where this is
the case. The builder is supposed to generate the configuration
file for a <link xlink:href='http://jetty.mortbay.org/'>Jetty
servlet container</link>. A servlet container contains a number
of servlets (<filename>*.war</filename> files) each exported under
a specific URI prefix. So the servlet configuration is a list of
attribute sets containing the <varname>path</varname> and
<varname>war</varname> of the servlet (<xref
linkend='ex-toxml-co-servlets' />). This kind of information is
difficult to communicate with the normal method of passing
information through an environment variable, which just
concatenates everything together into a string (which might just
work in this case, but wouldn’t work if fields are optional or
contain lists themselves). Instead the Nix expression is
converted to an XML representation with
<function>toXML</function>, which is unambiguous and can easily be
processed with the appropriate tools. For instance, in the
example an XSLT stylesheet (<xref linkend='ex-toxml-co-stylesheet'
/>) is applied to it (<xref linkend='ex-toxml-co-apply' />) to
generate the XML configuration file for the Jetty server. The XML
representation produced from <xref linkend='ex-toxml-co-servlets'
/> by <function>toXML</function> is shown in <xref
linkend='ex-toxml-result' />.</para>
<para>Note that <xref linkend='ex-toxml' /> uses the <function
linkend='builtin-toFile'>toFile</function> built-in to write the
builder and the stylesheet “inline” in the Nix expression. The
path of the stylesheet is spliced into the builder at
<literal>xsltproc ${stylesheet}
<replaceable>...</replaceable></literal>.</para>
<example xml:id='ex-toxml'><title>Passing information to a builder
using <function>toXML</function></title>
<programlisting><![CDATA[
{stdenv, fetchurl, libxslt, jira, uberwiki}:
stdenv.mkDerivation (rec {
name = "web-server";
buildInputs = [libxslt];
builder = builtins.toFile "builder.sh" "
source $stdenv/setup
mkdir $out
echo $servlets | xsltproc ${stylesheet} - > $out/server-conf.xml]]> <co xml:id='ex-toxml-co-apply' /> <![CDATA[
";
stylesheet = builtins.toFile "stylesheet.xsl"]]> <co xml:id='ex-toxml-co-stylesheet' /> <![CDATA[
"<?xml version='1.0' encoding='UTF-8'?>
<xsl:stylesheet xmlns:xsl='http://www.w3.org/1999/XSL/Transform' version='1.0'>
<xsl:template match='/'>
<Configure>
<xsl:for-each select='/expr/list/attrs'>
<Call name='addWebApplication'>
<Arg><xsl:value-of select=\"attr[@name = 'path']/string/@value\" /></Arg>
<Arg><xsl:value-of select=\"attr[@name = 'war']/path/@value\" /></Arg>
</Call>
</xsl:for-each>
</Configure>
</xsl:template>
</xsl:stylesheet>
";
servlets = builtins.toXML []]> <co xml:id='ex-toxml-co-servlets' /> <![CDATA[
{ path = "/bugtracker"; war = jira + "/lib/atlassian-jira.war"; }
{ path = "/wiki"; war = uberwiki + "/uberwiki.war"; }
];
})]]></programlisting>
</example>
<example xml:id='ex-toxml-result'><title>XML representation produced by
<function>toXML</function></title>
<programlisting><![CDATA[<?xml version='1.0' encoding='utf-8'?>
<expr>
<list>
<attrs>
<attr name="path">
<string value="/bugtracker" />
</attr>
<attr name="war">
<path value="/nix/store/d1jh9pasa7k2...-jira/lib/atlassian-jira.war" />
</attr>
</attrs>
<attrs>
<attr name="path">
<string value="/wiki" />
</attr>
<attr name="war">
<path value="/nix/store/y6423b1yi4sx...-uberwiki/uberwiki.war" />
</attr>
</attrs>
</list>
</expr>]]></programlisting>
</example>
</listitem>
</varlistentry>
<varlistentry><term><function>builtins.trace</function>
<replaceable>e1</replaceable> <replaceable>e2</replaceable></term>
<listitem><para>Evaluate <replaceable>e1</replaceable> and print its
abstract syntax representation on standard error. Then return
<replaceable>e2</replaceable>. This function is useful for
debugging.</para></listitem>
</varlistentry>
</variablelist>
</section>

117
doc/manual/conf-file.xml
View file

@ -118,6 +118,123 @@ env-keep-derivations = false
</varlistentry>
<varlistentry xml:id="conf-build-max-silent-time"><term><literal>build-max-silent-time</literal></term>
<listitem>
<para>This option defines the maximum number of seconds that a
builder can go without producing any data on standard output or
standard error. This is useful (for instance in a automated
build system) to catch builds that are stuck in an infinite
loop, or to catch remote builds that are hanging due to network
problems. It can be overriden using the <option
linkend="opt-max-silent-time">--max-silent-time</option> command
line switch.</para>
<para>The value <literal>0</literal> means that there is no
timeout. This is also the default.</para>
</listitem>
</varlistentry>
<varlistentry xml:id="conf-build-users-group"><term><literal>build-users-group</literal></term>
<listitem><para>This options specifies the Unix group containing
the Nix build user accounts. In multi-user Nix installations,
builds should not be performed by the Nix account since that would
allow users to arbitrarily modify the Nix store and database by
supplying specially crafted builders; and they cannot be performed
by the calling user since that would allow him/her to influence
the build result.</para>
<para>Therefore, if this option is non-empty and specifies a valid
group, builds will be performed under the user accounts that are a
member of the group specified here (as listed in
<filename>/etc/group</filename>). Those user accounts should not
be used for any other purpose!</para>
<para>Nix will never run two builds under the same user account at
the same time. This is to prevent an obvious security hole: a
malicious user writing a Nix expression that modifies the build
result of a legitimate Nix expression being built by another user.
Therefore it is good to have as many Nix build user accounts as
you can spare. (Remember: uids are cheap.)</para>
<para>The build users should have permission to create files in
the Nix store, but not delete them. Therefore,
<filename>/nix/store</filename> should be owned by the Nix
account, its group should be the group specified here, and its
mode should be <literal>1775</literal>.</para>
<para>If the build users group is empty, builds will be performed
under the uid of the Nix process (that is, the uid of the caller
if <envar>NIX_REMOTE</envar> is empty, the uid under which the Nix
daemon runs if <envar>NIX_REMOTE</envar> is
<literal>daemon</literal>, or the uid that owns the setuid
<command>nix-worker</command> program if <envar>NIX_REMOTE</envar>
is <literal>slave</literal>). Obviously, this should not be used
in multi-user settings with untrusted users.</para>
</listitem>
</varlistentry>
<varlistentry><term><literal>build-use-chroot</literal></term>
<listitem><para>If set to <literal>true</literal>, builds will be
performed in a <emphasis>chroot environment</emphasis>, i.e., the
build will be isolated from the normal file system hierarchy and
will only see the Nix store, the temporary build directory, and
the directories configured with the <link
linkend='conf-build-chroot-dirs'><literal>build-chroot-dirs</literal>
option</link> (such as <filename>/proc</filename> and
<filename>/dev</filename>). This is useful to prevent undeclared
dependencies on files in directories such as
<filename>/usr/bin</filename>.</para>
<para>The use of a chroot requires that Nix is run as root (but
you can still use the <link
linkend='conf-build-users-group'>“build users” feature</link> to
perform builds under different users than root). Currently,
chroot builds only work on Linux because Nix uses “bind mounts” to
make the Nix store and other directories available inside the
chroot.</para>
</listitem>
</varlistentry>
<varlistentry xml:id="conf-build-chroot-dirs"><term><literal>build-chroot-dirs</literal></term>
<listitem><para>When builds are performed in a chroot environment,
Nix will mount (using <command>mount --bind</command> on Linux)
some directories from the normal file system hierarchy inside the
chroot. These are the Nix store, the temporary build directory
(usually
<filename>/tmp/nix-<replaceable>pid</replaceable>-<replaceable>number</replaceable></filename>)
and the directories listed here. The default is <literal>dev
/proc</literal>. Files in <filename>/dev</filename> (such as
<filename>/dev/null</filename>) are needed by many builds, and
some files in <filename>/proc</filename> may also be needed
occasionally.</para>
<para>The value used on NixOS is
<programlisting>
build-use-chroot = /dev /proc /bin</programlisting>
to make the <filename>/bin/sh</filename> symlink available (which
is still needed by many builders).</para>
</listitem>
</varlistentry>
<varlistentry><term><literal>system</literal></term>
<listitem><para>This option specifies the canonical Nix system

404
doc/manual/introduction.xml
View file

@ -1,135 +1,304 @@
<chapter xmlns="http://docbook.org/ns/docbook"
xmlns:xlink="http://www.w3.org/1999/xlink">
xmlns:xlink="http://www.w3.org/1999/xlink"
xml:id="chap-introduction">
<title>Introduction</title>
<para>Nix is a system for the deployment of software. Software
deployment is concerned with the creation, distribution, and
management of software components (<quote>packages</quote>). Its main
features are:
<itemizedlist>
<section><title>About Nix</title>
<listitem><para>It helps you make sure that dependency specifications
are complete. In general in a deployment system you have to specify
for each component what its dependencies are, but there are no
guarantees that this specification is complete. If you forget a
dependency, then the component will build and work correctly on
<emphasis>your</emphasis> machine if you have the dependency
installed, but not on the end user's machine if it's not
there.</para></listitem>
<para>Nix is a <emphasis>purely functional package manager</emphasis>.
This means that it treats packages like values in purely functional
programming languages such as Haskell — they are built by functions
that don’t have side-effects, and they never change after they have
been built. Nix stores packages in the <emphasis>Nix
store</emphasis>, usually the directory
<filename>/nix/store</filename>, where each package has its own unique
subdirectory such as
<listitem><para>It is possible to have <emphasis>multiple versions or
variants</emphasis> of a component installed at the same time. In
contrast, in systems such as RPM different versions of the same
package tend to install to the same location in the file system, so
installing one version will remove the other. This is especially
important if you want to use applications that have conflicting
requirements on different versions of a component (e.g., application A
requires version 1.0 of library X, while application B requires a
non-backwards compatible version 1.1).</para></listitem>
<programlisting>
/nix/store/r8vvq9kq18pz08v249h8my6r9vs7s0n3-firefox-2.0.0.1/
</programlisting>
<listitem><para>Users can have different <quote>views</quote>
(<quote>profiles</quote> in Nix parlance) on the set of installed
applications in a system. For instance, one user can have version 1.0
of some package visible, while another is using version 1.1, and a
third doesn't use it at all.</para></listitem>
where <literal>r8vvq9kq…</literal> is a unique identifier for the
package that captures all its dependencies (it’s a cryptographic hash
of the package’s build dependency graph). This enables many powerful
features.</para>
<listitem><para>It is possible to atomically
<emphasis>upgrade</emphasis> software. I.e., there is no time window
during an upgrade in which part of the old version and part of the new
version are simultaneously visible (which might well cause the
component to fail).</para></listitem>
<listitem><para>Likewise, it is possible to atomically roll back after
an install, upgrade, or uninstall action. That is, in a fast (O(1))
operation the previous configuration of the system can be restored.
This is because upgrade or uninstall actions don't actually remove
components from the system.</para></listitem>
<simplesect><title>Multiple versions</title>
<listitem><para>Unused components can be
<emphasis>garbage-collected</emphasis> automatically and safely: when
you remove an application from a profile, its dependencies will be
deleted by the garbage collector only if there are no other active
applications using them.</para></listitem>
<para>You can have multiple versions or variants of a package
installed at the same time. This is especially important when
different applications have dependencies on different versions of the
same package — it prevents the “DLL hell”. Because of the hashing
scheme, different versions of a package end up in different paths in
the Nix store, so they don’t interfere with each other.</para>
<listitem><para>Nix supports both source-based deployment models
(where you distribute <emphasis>Nix expressions</emphasis> that tell
Nix how to build software from source) and binary-based deployment
models. The latter is more-or-less transparent: installation of
components is always based on Nix expressions, but if the expressions
have been built before and Nix knows that the resulting binaries are
available somewhere, it will use those instead.</para></listitem>
<para>An important consequence is that operations like upgrading or
uninstalling an application cannot break other applications, since
these operations never “destructively” update or delete files that are
used by other packages.</para>
<listitem><para>Nix is flexible in the deployment policies that it
supports. There is a clear separation between the tools that
implement basic Nix <emphasis>mechanisms</emphasis> (e.g., building
Nix expressions), and the tools that implement various deployment
<emphasis>policies</emphasis>. For instance, there is a concept of
<quote>Nix channels</quote> that can be used to keep software
installations up-to-date automatically from a network source. This is
a policy that is implemented by a fairly short Perl script, which can
be adapted easily to achieve similar policies.</para></listitem>
</simplesect>
<listitem><para>Nix component builds aim to be <quote>pure</quote>;
that is, unaffected by anything other than the declared dependencies.
This means that if a component was built successfully once, it can be
rebuilt again on another machine and the result will be the same. We
cannot <emphasis>guarantee</emphasis> this (e.g., if the build depends
on the time-of-day), but Nix (and the tools in the Nix Packages
collection) takes special care to help achieve this.</para></listitem>
<listitem><para>Nix expressions (the things that tell Nix how to build
components) are self-contained: they describe not just components but
complete compositions. In other words, Nix expressions also describe
how to build all the dependencies. This is in contrast to component
specification languages like RPM spec files, which might say that a
component X depends on some other component Y, but since it does not
describe <emphasis>exactly</emphasis> what Y is, the result of
building or running X might be different on different machines.
Combined with purity, self-containedness ensures that a component that
<quote>works</quote> on one machine also works on another, when
deployed using Nix.</para></listitem>
<simplesect><title>Complete dependencies</title>
<listitem><para>The Nix expression language makes it easy to describe
variability in components (e.g., optional features or
dependencies).</para></listitem>
<para>Nix helps you make sure that package dependency specifications
are complete. In general, when you’re making a package for a package
management system like RPM, you have to specify for each package what
its dependencies are, but there are no guarantees that this
specification is complete. If you forget a dependency, then the
component will build and work correctly on <emphasis>your</emphasis>
machine if you have the dependency installed, but not on the end
user's machine if it's not there.</para>
<listitem><para>Nix is ideal for building build farms that do
continuous builds of software from a version management system, since
it can take care of building all the dependencies as well. Also, Nix
only rebuilds components that have changed, so there are no
unnecessary builds. In addition, Nix can transparently distribute
build jobs over different machines, including different
platforms.</para></listitem>
<para>Since Nix on the other hand doesn’t install packages in “global”
locations like <filename>/usr/bin</filename> but in package-specific
directories, the risk of incomplete dependencies is greatly reduced.
This is because tools such as compilers don’t search in per-packages
directories such as
<filename>/nix/store/5lbfaxb722zp…-openssl-0.9.8d/include</filename>,
so if a package builds correctly on your system, this is because you
specified the dependency explicitly.</para>
<listitem><para>Nix can be used not only for software deployment, but
also for <emphasis>service deployment</emphasis>, such as the
deployment of a complete web server with all its configuration files,
static pages, software dependencies, and so on. Nix's advantages for
software deployment also apply here: for instance, the ability
trivially to have multiple configurations at the same time, or the
ability to do rollbacks.</para></listitem>
<para>Runtime dependencies are found by scanning binaries for the hash
parts of Nix store paths (such as <literal>r8vvq9kq…</literal>). This
sounds risky, but it works extremely well.</para>
<listitem><para>Nix can efficiently upgrade between different versions
of a component through <emphasis>binary patching</emphasis>. If
patches are available on a server, and you try to install a new
version of some component, Nix will automatically apply a patch (or
sequence of patches), if available, to transform the installed
component into the new version.</para></listitem>
</simplesect>
</itemizedlist>
</para>
<simplesect><title>Multi-user support</title>
<para>Starting at version 0.11, Nix has multi-user support. This
means that non-privileged users can securely install software. Each
user can have a different <emphasis>profile</emphasis>, a set of
packages in the Nix store that appear in the user’s
<envar>PATH</envar>. If a user installs a package that another user
has already installed previously, the package won’t be built or
downloaded a second time. At the same time, it is not possible for
one user to inject a Trojan horse into a package that might be used by
another user.</para>
<!--
<para>More details can be found in Section 3 of our <a
href="docs/papers.html#securesharing">ASE 2005 paper</a>.</para>
-->
</simplesect>
<simplesect><title>Atomic upgrades and rollbacks</title>
<para>Since package management operations never overwrite packages in
the Nix store but just add new versions in different paths, they are
<emphasis>atomic</emphasis>. So during a package upgrade, there is no
time window in which the package has some files from the old version
and some files from the new version — which would be bad because a
program might well crash if it’s started during that period.</para>
<para>And since package aren’t overwritten, the old versions are still
there after an upgrade. This means that you can <emphasis>roll
back</emphasis> to the old version:</para>
<screen>
$ nix-env --upgrade <replaceable>some-packages</replaceable>
$ nix-env --rollback
</screen>
</simplesect>
<simplesect><title>Garbage collection</title>
<para>When you install a package like this…
<screen>
$ nix-env --uninstall firefox
</screen>
the package isn’t deleted from the system right away (after all, you
might want to do a rollback, or it might be in the profiles of other
users). Instead, unused packages can be deleted safely by running the
<emphasis>garbage collector</emphasis>:
<screen>
$ nix-collect-garbage
</screen>
This deletes all packages that aren’t in use by any user profile or by
a currently running program.</para>
</simplesect>
<simplesect><title>Functional package language</title>
<para>Packages are built from <emphasis>Nix expressions</emphasis>,
which is a simple functional language. A Nix expression describes
everything that goes into a package build action (a “derivation”):
other packages, sources, the build script, environment variables for
the build script, etc. Nix tries very hard to ensure that Nix
expressions are <emphasis>deterministic</emphasis>: building a Nix
expression twice should yield the same result.</para>
<para>Because it’s a functional language, it’s easy to support
building variants of a package: turn the Nix expression into a
function and call it any number of times with the appropriate
arguments. Due to the hashing scheme, variants don’t conflict with
each other in the Nix store.</para>
</simplesect>
<simplesect><title>Transparent source/binary deployment</title>
<para>Nix expressions generally describe how to build a package from
source, so an installation action like
<screen>
$ nix-env --install firefox
</screen>
<emphasis>could</emphasis> cause quite a bit of build activity, as not
only Firefox but also all its dependencies (all the way up to the C
library and the compiler) would have to built, at least if they are
not already in the Nix store. This is a <emphasis>source deployment
model</emphasis>. For most users, building from source is not very
pleasant as it takes far too long. However, Nix can automatically
skip building from source and download a pre-built binary instead if
it knows about it. <emphasis>Nix channels</emphasis> provide Nix
expressions along with pre-built binaries.</para>
<!--
<para>source deployment model (like <a
href="http://www.gentoo.org/">Gentoo</a>) and a binary model (like
RPM)</para>
-->
</simplesect>
<simplesect><title>Binary patching</title>
<para>In addition to downloading binaries automatically if they’re
available, Nix can download binary deltas that patch an existing
package in the Nix store into a new version. This speeds up
upgrades.</para>
</simplesect>
<simplesect><title>Nix Packages collection</title>
<para>We provide a large set of Nix expressions containing hundreds of
existing Unix packages, the <emphasis>Nix Packages
collection</emphasis> (Nixpkgs).</para>
</simplesect>
<simplesect><title>Service deployment</title>
<para>Nix can be used not only for rolling out packages, but also
complete <emphasis>configurations</emphasis> of services. This is
done by treating all the static bits of a service (such as software
packages, configuration files, control scripts, static web pages,
etc.) as “packages” that can be built by Nix expressions. As a
result, all the features above apply to services as well: for
instance, you can roll back a web server configuration if a
configuration change turns out to be undesirable, you can easily have
multiple instances of a service (e.g., a test and production server),
and because the whole service is built in a purely functional way from
a Nix expression, it is repeatable so you can easily reproduce the
service on another machine.</para>
<!--
<para>You can read more about this in our <a
href="docs/papers.html#servicecm">SCM-12 paper</a>.</para>
-->
</simplesect>
<simplesect><title>Portability</title>
<para>Nix should run on most Unix systems, including Linux, FreeBSD and
Mac OS X. It is also supported on Windows using Cygwin.</para>
</simplesect>
<simplesect><title>NixOS</title>
<para>NixOS is a Linux distribution based on Nix. It uses Nix not
just for package management but also to manage the system
configuration (e.g., to build configuration files in
<filename>/etc</filename>). This means, among other things, that it’s
possible to easily roll back the entire configuration of the system to
an earlier state. Also, users can install software without root
privileges. For more information and downloads, see the <link
xlink:href="http://nix.cs.uu.nl/nixos/">NixOS homepage</link>.</para>
</simplesect>
<!-- other features:
- build farms
- reproducibility (Nix expressions allows whole configuration to be rebuilt)
-->
</section>
<section><title>About us</title>
<para>Nix was developed at the <link
xlink:href="http://www.cs.uu.nl/">Department of Information and
Computing Sciences</link>, Utrecht University by the <link
xlink:href="http://www.cs.uu.nl/wiki/Trace/WebHome">TraCE
project</link>. The project is funded by the Software Engineering
Research Program <link
xlink:href="http://www.jacquard.nl/">Jacquard</link> to improve the
support for variability in software systems.</para>
</section>
<section><title>About this manual</title>
<para>This manual tells you how to install and use Nix and how to
write Nix expressions for software not already in the Nix Packages
collection. It also discusses some advanced topics, such as setting
up a Nix-based build farm, and doing service deployment using
Nix.</para>
up a Nix-based build farm.</para>
<note><para>Some background information on Nix can be found in a
number of papers. The ICSE 2004 paper <citetitle
</section>
<section><title>License</title>
<para>Nix is free software; you can redistribute it and/or modify it
under the terms of the <link
xlink:href="http://www.gnu.org/licenses/lgpl.html">GNU Lesser General
Public License</link> as published by the <link
xlink:href="http://www.fsf.org/">Free Software Foundation</link>;
either version 2.1 of the License, or (at your option) any later
version. Nix is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.</para>
</section>
<section><title>More information</title>
<para>Some background information on Nix can be found in a number of
papers. The ICSE 2004 paper <citetitle
xlink:href='http://www.cs.uu.nl/~eelco/pubs/immdsd-icse2004-final.pdf'>Imposing
a Memory Management Discipline on Software Deployment</citetitle>
discusses the hashing mechanism used to ensure reliable dependency
@ -141,10 +310,27 @@ gives a more general discussion of Nix from a system-administration
perspective. The CBSE 2005 paper <citetitle
xlink:href='http://www.cs.uu.nl/~eelco/pubs/eupfcdm-cbse2005-final.pdf'>Efficient
Upgrading in a Purely Functional Component Deployment Model
</citetitle> is about transparent patch deployment in Nix. Finally,
the SCM-12 paper <citetitle
</citetitle> is about transparent patch deployment in Nix. The SCM-12
paper <citetitle
xlink:href='http://www.cs.uu.nl/~eelco/pubs/servicecm-scm12-final.pdf'>
Service Configuration Management</citetitle> shows how services (e.g.,
web servers) can be deployed and managed through Nix.</para></note>
web servers) can be deployed and managed through Nix. A short
overview of NixOS is given in the HotOS XI paper <citetitle
xlink:href="http://www.cs.uu.nl/~eelco/pubs/hotos-final.pdf">Purely
Functional System Configuration Management</citetitle>. The Nix
homepage has <link
xlink:href="http://nix.cs.uu.nl/docs/papers.html">an up-to-date list
of Nix-related papers</link>.</para>
<para>Nix is the subject of Eelco Dolstra’s PhD thesis <citetitle
xlink:href="http://igitur-archive.library.uu.nl/dissertations/2006-0118-200031/index.htm">The
Purely Functional Software Deployment Model</citetitle>, which
contains most of the papers listed above.</para>
<para>Nix has a homepage at <link
xlink:href="http://nix.cs.uu.nl/"/>.</para>
</section>
</chapter>

4
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View file

@ -13,6 +13,10 @@
</group>
<replaceable>number</replaceable>
</arg>
<arg>
<arg><option>--max-silent-time</option></arg>
<replaceable>number</replaceable>
</arg>
<arg><option>--keep-going</option></arg>
<arg><option>-k</option></arg>
<arg><option>--keep-failed</option></arg>

11
doc/manual/opt-common.xml
View file

@ -103,6 +103,17 @@
</varlistentry>
<varlistentry xml:id="opt-max-silent-time"><term><option>--max-silent-time</option></term>
<listitem><para>Sets the maximum number of seconds that a builder
can go without producing any data on standard output or standard
error. The default is specified by the <link
linkend='conf-build-max-silent-time'><literal>build-max-silent-time</literal></link>
configuration setting. <literal>0</literal> means no
time-out.</para></listitem>
</varlistentry>
<varlistentry><term><option>--keep-going</option></term>
<term><option>-k</option></term>

6
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View file

@ -92,7 +92,8 @@
output is already in the Nix store or that can be substituted (i.e.,
downloaded from somewhere). In other words, it shows the packages
that can be installed “quickly”, i.e., don’t need to be built from
source.</para></listitem>
source. TODO: flag is also available in nix-env -i /
-u.</para></listitem>
<listitem><para>TODO: new built-ins
@ -116,6 +117,9 @@
limited form of caching. This is used by
<command>nix-channel</command> to prevent unnecessary downloads when
the channel hasn’t changed.</para></listitem>
<listitem><para>TODO: chroot support.</para></listitem>
</itemizedlist>

5
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@ -46,6 +46,11 @@ h3 /* subsections */
font-size: 125%;
}
div.simplesect h2
{
font-size: 110%;
}
div.appendix h3
{
font-size: 150%;

586
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View file

@ -1,6 +1,7 @@
<chapter xmlns="http://docbook.org/ns/docbook"
xmlns:xlink="http://www.w3.org/1999/xlink"
xml:id='chap-writing-nix-expressions'>
xml:id='chap-writing-nix-expressions'
xmlns:xi="http://www.w3.org/2001/XInclude">
<title>Writing Nix Expressions</title>
@ -1339,589 +1340,8 @@ command-line argument. See <xref linkend='sec-standard-environment'
</section>
<section><title>Built-in functions</title>
<para>This section lists the functions and constants built into the
Nix expression evaluator. (The built-in function
<function>derivation</function> is discussed above.) Some built-ins,
such as <function>derivation</function>, are always in scope of every
Nix expression; you can just access them right away. But to prevent
polluting the namespace too much, most built-ins are not in scope.
Instead, you can access them through the <varname>builtins</varname>
built-in value, which is an attribute set that contains all built-in
functions and values. For instance, <function>derivation</function>
is also available as <function>builtins.derivation</function>.</para>
<variablelist>
<varlistentry><term><function>abort</function> <replaceable>s</replaceable></term>
<listitem><para>Abort Nix expression evaluation, print error
message <replaceable>s</replaceable>.</para></listitem>
</varlistentry>
<varlistentry><term><function>builtins.add</function>
<replaceable>e1</replaceable> <replaceable>e2</replaceable></term>
<listitem><para>Return the sum of the integers
<replaceable>e1</replaceable> and
<replaceable>e2</replaceable>.</para></listitem>
</varlistentry>
<varlistentry><term><function>builtins.attrNames</function>
<replaceable>attrs</replaceable></term>
<listitem><para>Return the names of the attributes in the
attribute set <replaceable>attrs</replaceable> in a sorted list.
For instance, <literal>builtins.attrNames {y = 1; x =
"foo";}</literal> evaluates to <literal>["x" "y"]</literal>.
There is no built-in function <function>attrValues</function>, but
you can easily define it yourself:
<programlisting>
attrValues = attrs: map (name: builtins.getAttr name attrs) (builtins.attrNames attrs);</programlisting>
</para></listitem>
</varlistentry>
<varlistentry><term><function>baseNameOf</function> <replaceable>s</replaceable></term>
<listitem><para>Return the <emphasis>base name</emphasis> of the
string <replaceable>s</replaceable>, that is, everything following
the final slash in the string. This is similar to the GNU
<command>basename</command> command.</para></listitem>
</varlistentry>
<varlistentry><term><varname>builtins</varname></term>
<listitem><para>The attribute set <varname>builtins</varname>
contains all the built-in functions and values. You can use
<varname>builtins</varname> to test for the availability of
features in the Nix installation, e.g.,
<programlisting>
if builtins ? getEnv then builtins.getEnv "PATH" else ""</programlisting>
This allows a Nix expression to fall back gracefully on older Nix
installations that don’t have the desired built-in function.
However, in that case you should not write
<programlisting>
if builtins ? getEnv then __getEnv "PATH" else ""</programlisting>
This Nix expression will trigger an “undefined variable” error on
older Nix versions since <function>__getEnv</function> doesn’t
exist. <literal>builtins.getEnv</literal>, on the other hand, is
safe since <literal>builtins</literal> always exists and attribute
selection is lazy, so it’s only performed if the test
succeeds.</para></listitem>
</varlistentry>
<varlistentry
xml:id='builtin-currentSystem'><term><varname>builtins.currentSystem</varname></term>
<listitem><para>The built-in value <varname>currentSystem</varname>
evaluates to the Nix platform identifier for the Nix installation
on which the expression is being evaluated, such as
<literal>"i686-linux"</literal> or
<literal>"powerpc-darwin"</literal>.</para></listitem>
</varlistentry>
<!--
<varlistentry><term><function>currentTime</function></term>
<listitem><para>The built-in value <varname>currentTime</varname>
returns the current system time in seconds since 00:00:00 1/1/1970
UTC. Due to the evaluation model of Nix expressions
(<emphasis>maximal laziness</emphasis>), it always yields the same
value within an execution of Nix.</para></listitem>
</varlistentry>
-->
<!--
<varlistentry><term><function>dependencyClosure</function></term>
<listitem><para>TODO</para></listitem>
</varlistentry>
-->
<varlistentry><term><function>derivation</function>
<replaceable>attrs</replaceable></term>
<listitem><para><function>derivation</function> is described in
<xref linkend='ssec-derivation' />.</para></listitem>
</varlistentry>
<varlistentry><term><function>dirOf</function> <replaceable>s</replaceable></term>
<listitem><para>Return the directory part of the string
<replaceable>s</replaceable>, that is, everything before the final
slash in the string. This is similar to the GNU
<command>dirname</command> command.</para></listitem>
</varlistentry>
<varlistentry><term><function>builtins.getAttr</function>
<replaceable>s</replaceable> <replaceable>attrs</replaceable></term>
<listitem><para><function>getAttr</function> returns the attribute
named <replaceable>s</replaceable> from the attribute set
<replaceable>attrs</replaceable>. Evaluation aborts if the
attribute doesn’t exist. This is a dynamic version of the
<literal>.</literal> operator, since <replaceable>s</replaceable>
is an expression rather than an identifier.</para></listitem>
</varlistentry>
<varlistentry><term><function>builtins.getEnv</function>
<replaceable>s</replaceable></term>
<listitem><para><function>getEnv</function> returns the value of
the environment variable <replaceable>s</replaceable>, or an empty
string if the variable doesn’t exist. This function should be
used with care, as it can introduce all sorts of nasty environment
dependencies in your Nix expression.</para>
<para><function>getEnv</function> is used in Nix Packages to
locate the file <filename>~/.nixpkgs/config.nix</filename>, which
contains user-local settings for Nix Packages. (That is, it does
a <literal>getEnv "HOME"</literal> to locate the user’s home
directory.)</para></listitem>
</varlistentry>
<varlistentry><term><function>builtins.hasAttr</function>
<replaceable>s</replaceable> <replaceable>attrs</replaceable></term>
<listitem><para><function>hasAttr</function> returns
<literal>true</literal> if the attribute set
<replaceable>attrs</replaceable> has an attribute named
<replaceable>s</replaceable>, and <literal>false</literal>
otherwise. This is a dynamic version of the <literal>?</literal>
operator, since <replaceable>s</replaceable> is an expression
rather than an identifier.</para></listitem>
</varlistentry>
<varlistentry><term><function>builtins.head</function>
<replaceable>list</replaceable></term>
<listitem><para>Return the first element of a list; abort
evaluation if the argument isn’t a list or is an empty list. You
can test whether a list is empty by comparing it with
<literal>[]</literal>.</para></listitem>
</varlistentry>
<varlistentry><term><function>import</function>
<replaceable>path</replaceable></term>
<listitem><para>Load, parse and return the Nix expression in the
file <replaceable>path</replaceable>. Evaluation aborts if the
file doesn’t exist or contains an incorrect Nix
expression. <function>import</function> implements Nix’s module
system: you can put any Nix expression (such as an attribute set
or a function) in a separate file, and use it from Nix expressions
in other files.</para>
<para>A Nix expression loaded by <function>import</function> must
not contain any <emphasis>free variables</emphasis> (identifiers
that are not defined in the Nix expression itself and are not
built-in). Therefore, it cannot refer to variables that are in
scope at the call site. For instance, if you have a calling
expression
<programlisting>
rec {
x = 123;
y = import ./foo.nix;
}</programlisting>
then the following <filename>foo.nix</filename> will give an
error:
<programlisting>
x + 456</programlisting>
since <varname>x</varname> is not in scope in
<filename>foo.nix</filename>. If you want <varname>x</varname>
to be available in <filename>foo.nix</filename>, you should pass
it as a function argument:
<programlisting>
rec {
x = 123;
y = import ./foo.nix x;
}</programlisting>
and
<programlisting>
x: x + 456</programlisting>
(The function argument doesn’t have to be called
<varname>x</varname> in <filename>foo.nix</filename>; any name
would work.)</para></listitem>
</varlistentry>
<varlistentry><term><function>builtins.isList</function>
<replaceable>e</replaceable></term>
<listitem><para>Return <literal>true</literal> if
<replaceable>e</replaceable> evaluates to a list, and
<literal>false</literal> otherwise.</para></listitem>
</varlistentry>
<varlistentry><term><function>isNull</function>
<replaceable>e</replaceable></term>
<listitem><para>Return <literal>true</literal> if
<replaceable>e</replaceable> evaluates to <literal>null</literal>,
and <literal>false</literal> otherwise.</para>
<warning><para>This function is <emphasis>deprecated</emphasis>;
just write <literal>e == null</literal> instead.</para></warning>
</listitem>
</varlistentry>
<varlistentry><term><function>builtins.lessThan</function>
<replaceable>e1</replaceable> <replaceable>e2</replaceable></term>
<listitem><para>Return <literal>true</literal> if the integer
<replaceable>e1</replaceable> is less than the integer
<replaceable>e2</replaceable>, and <literal>false</literal>
otherwise. Evaluation aborts if either
<replaceable>e1</replaceable> or <replaceable>e2</replaceable>
does not evaluate to an integer.</para></listitem>
</varlistentry>
<varlistentry><term><function>map</function>
<replaceable>f</replaceable> <replaceable>list</replaceable></term>
<listitem><para>Apply the function <replaceable>f</replaceable> to
each element in the list <replaceable>list</replaceable>. For
example,
<programlisting>
map (x: "foo" + x) ["bar" "bla" "abc"]</programlisting>
evaluates to <literal>["foobar" "foobla"
"fooabc"]</literal>.</para></listitem>
</varlistentry>
<varlistentry><term><function>builtins.pathExists</function>
<replaceable>path</replaceable></term>
<listitem><para>Return <literal>true</literal> if the path
<replaceable>path</replaceable> exists, and
<literal>false</literal> otherwise. One application of this
function is to conditionally include a Nix expression containing
user configuration:
<programlisting>
let
fileName = builtins.getEnv "CONFIG_FILE";
config =
if fileName != "" &amp;&amp; builtins.pathExists (builtins.toPath fileName)
then import (builtins.toPath fileName)
else { someSetting = false; }; <lineannotation># default configuration</lineannotation>
in config.someSetting</programlisting>
(Note that <envar>CONFIG_FILE</envar> must be an absolute path for
this to work.)</para></listitem>
</varlistentry>
<!--
<varlistentry><term><function>relativise</function></term>
<listitem><para>TODO</para></listitem>
</varlistentry>
-->
<varlistentry><term><function>removeAttrs</function>
<replaceable>attrs</replaceable> <replaceable>list</replaceable></term>
<listitem><para>Remove the attributes listed in
<replaceable>list</replaceable> from the attribute set
<replaceable>attrs</replaceable>. The attributes don’t have to
exist in <replaceable>attrs</replaceable>. For instance,
<screen>
removeAttrs { x = 1; y = 2; z = 3; } ["a" "x" "z"]</screen>
evaluates to <literal>{y = 2;}</literal>.</para></listitem>
</varlistentry>
<varlistentry><term><function>builtins.tail</function>
<replaceable>list</replaceable></term>
<listitem><para>Return the second to last elements of a list;
abort evaluation if the argument isn’t a list or is an empty
list.</para></listitem>
</varlistentry>
<varlistentry
xml:id='builtin-toFile'><term><function>builtins.toFile</function>
<replaceable>name</replaceable> <replaceable>s</replaceable></term>
<listitem><para>Store the string <replaceable>s</replaceable> in a
file in the Nix store and return its path. The file has suffix
<replaceable>name</replaceable>. This file can be used as an
input to derivations. One application is to write builders
“inline”. For instance, the following Nix expression combines
<xref linkend='ex-hello-nix' /> and <xref
linkend='ex-hello-builder' /> into one file:
<programlisting>
{stdenv, fetchurl, perl}:
stdenv.mkDerivation {
name = "hello-2.1.1";
builder = builtins.toFile "builder.sh" "
source $stdenv/setup
PATH=$perl/bin:$PATH
tar xvfz $src
cd hello-*
./configure --prefix=$out
make
make install
";
src = fetchurl {
url = http://nix.cs.uu.nl/dist/tarballs/hello-2.1.1.tar.gz;
md5 = "70c9ccf9fac07f762c24f2df2290784d";
};
inherit perl;
}</programlisting>
</para>
<para>It is even possible for one file to refer to another, e.g.,
<programlisting>
builder = let
configFile = builtins.toFile "foo.conf" "
# This is some dummy configuration file.
<replaceable>...</replaceable>
";
in builtins.toFile "builder.sh" "
source $stdenv/setup
<replaceable>...</replaceable>
cp ${configFile} $out/etc/foo.conf
";</programlisting>
Note that <literal>${configFile}</literal> is an antiquotation
(see <xref linkend='ssec-values' />), so the result of the
expression <literal>configFile</literal> (i.e., a path like
<filename>/nix/store/m7p7jfny445k...-foo.conf</filename>) will be
spliced into the resulting string.</para>
<para>It is however <emphasis>not</emphasis> allowed to have files
mutually referring to each other, like so:
<programlisting>
let
foo = builtins.toFile "foo" "...${bar}...";
bar = builtins.toFile "bar" "...${foo}...";
in foo</programlisting>
This is not allowed because it would cause a cyclic dependency in
the computation of the cryptographic hashes for
<varname>foo</varname> and <varname>bar</varname>.</para></listitem>
</varlistentry>
<varlistentry><term><function>builtins.toPath</function> <replaceable>s</replaceable></term>
<listitem><para>Convert the string value
<replaceable>s</replaceable> into a path value. The string
<replaceable>s</replaceable> must represent an absolute path
(i.e., must start with <literal>/</literal>). The path need not
exist. The resulting path is canonicalised, e.g.,
<literal>builtins.toPath "//foo/xyzzy/../bar/"</literal> returns
<literal>/foo/bar</literal>.</para></listitem>
</varlistentry>
<varlistentry><term><function>toString</function> <replaceable>e</replaceable></term>
<listitem><para>Convert the expression
<replaceable>e</replaceable> to a string.
<replaceable>e</replaceable> can be a string (in which case
<function>toString</function> is a no-op) or a path (e.g.,
<literal>toString /foo/bar</literal> yields
<literal>"/foo/bar"</literal>.</para></listitem>
</varlistentry>
<varlistentry xml:id='builtin-toXML'><term><function>builtins.toXML</function> <replaceable>e</replaceable></term>
<listitem><para>Return a string containing an XML representation
of <replaceable>e</replaceable>. The main application for
<function>toXML</function> is to communicate information with the
builder in a more structured format than plain environment
variables.</para>
<!-- TODO: more formally describe the schema of the XML
representation -->
<para><xref linkend='ex-toxml' /> shows an example where this is
the case. The builder is supposed to generate the configuration
file for a <link xlink:href='http://jetty.mortbay.org/'>Jetty
servlet container</link>. A servlet container contains a number
of servlets (<filename>*.war</filename> files) each exported under
a specific URI prefix. So the servlet configuration is a list of
attribute sets containing the <varname>path</varname> and
<varname>war</varname> of the servlet (<xref
linkend='ex-toxml-co-servlets' />). This kind of information is
difficult to communicate with the normal method of passing
information through an environment variable, which just
concatenates everything together into a string (which might just
work in this case, but wouldn’t work if fields are optional or
contain lists themselves). Instead the Nix expression is
converted to an XML representation with
<function>toXML</function>, which is unambiguous and can easily be
processed with the appropriate tools. For instance, in the
example an XSLT stylesheet (<xref linkend='ex-toxml-co-stylesheet'
/>) is applied to it (<xref linkend='ex-toxml-co-apply' />) to
generate the XML configuration file for the Jetty server. The XML
representation produced from <xref linkend='ex-toxml-co-servlets'
/> by <function>toXML</function> is shown in <xref
linkend='ex-toxml-result' />.</para>
<para>Note that <xref linkend='ex-toxml' /> uses the <function
linkend='builtin-toFile'>toFile</function> built-in to write the
builder and the stylesheet “inline” in the Nix expression. The
path of the stylesheet is spliced into the builder at
<literal>xsltproc ${stylesheet}
<replaceable>...</replaceable></literal>.</para>
<example xml:id='ex-toxml'><title>Passing information to a builder
using <function>toXML</function></title>
<programlisting><![CDATA[
{stdenv, fetchurl, libxslt, jira, uberwiki}:
stdenv.mkDerivation (rec {
name = "web-server";
buildInputs = [libxslt];
builder = builtins.toFile "builder.sh" "
source $stdenv/setup
mkdir $out
echo $servlets | xsltproc ${stylesheet} - > $out/server-conf.xml]]> <co xml:id='ex-toxml-co-apply' /> <![CDATA[
";
stylesheet = builtins.toFile "stylesheet.xsl"]]> <co xml:id='ex-toxml-co-stylesheet' /> <![CDATA[
"<?xml version='1.0' encoding='UTF-8'?>
<xsl:stylesheet xmlns:xsl='http://www.w3.org/1999/XSL/Transform' version='1.0'>
<xsl:template match='/'>
<Configure>
<xsl:for-each select='/expr/list/attrs'>
<Call name='addWebApplication'>
<Arg><xsl:value-of select=\"attr[@name = 'path']/string/@value\" /></Arg>
<Arg><xsl:value-of select=\"attr[@name = 'war']/path/@value\" /></Arg>
</Call>
</xsl:for-each>
</Configure>
</xsl:template>
</xsl:stylesheet>
";
servlets = builtins.toXML []]> <co xml:id='ex-toxml-co-servlets' /> <![CDATA[
{ path = "/bugtracker"; war = jira + "/lib/atlassian-jira.war"; }
{ path = "/wiki"; war = uberwiki + "/uberwiki.war"; }
];
})]]></programlisting>
</example>
<example xml:id='ex-toxml-result'><title>XML representation produced by
<function>toXML</function></title>
<programlisting><![CDATA[<?xml version='1.0' encoding='utf-8'?>
<expr>
<list>
<attrs>
<attr name="path">
<string value="/bugtracker" />
</attr>
<attr name="war">
<path value="/nix/store/d1jh9pasa7k2...-jira/lib/atlassian-jira.war" />
</attr>
</attrs>
<attrs>
<attr name="path">
<string value="/wiki" />
</attr>
<attr name="war">
<path value="/nix/store/y6423b1yi4sx...-uberwiki/uberwiki.war" />
</attr>
</attrs>
</list>
</expr>]]></programlisting>
</example>
</listitem>
</varlistentry>
</variablelist>
</section>
<xi:include href="builtins.xml" />
</section>