Writing Nix Expressions

A simple Nix expression This section shows how to add and test the GNU Hello package to the Nix Packages collection. Hello is a program that prints out the text Hello, world!. To add a component to the Nix Packages collection, you generally need to do three things: Write a Nix expression for the component. This is a file that describes all the inputs involved in building the component, such as dependencies (other components required by the component), sources, and so on. Write a builder. This is a shell scriptIn fact, it can be written in any language, but typically it's a bash shell script. that actually builds the component from the inputs. Add the component to the file pkgs/top-level/all-packages.nix. The Nix expression written in the first step is a function; it requires other components 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 component.

The Nix expression Nix expression for GNU Hello (<filename>default.nix</filename>) {stdenv, fetchurl, perl}: stdenv.mkDerivation { name = "hello-2.1.1"; builder = ./builder.sh; src = fetchurl { url = ftp://ftp.nluug.nl/pub/gnu/hello/hello-2.1.1.tar.gz; md5 = "70c9ccf9fac07f762c24f2df2290784d"; }; inherit perl; } shows a Nix expression for GNU Hello. It's actually already in the Nix Packages collection in pkgs/applications/misc/hello/ex-1/default.nix. It is customary to place each package in a separate directory and call the single Nix expression in that directory default.nix. The file has the following elements (referenced from the figure by number): This states that the expression is a function that expects to be called with three arguments: stdenv, fetchurl, and perl. They are needed to build Hello, but we don't know how to build them here; that's why they are function arguments. stdenv is a component that is used by almost all Nix Packages components; it provides a standard 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 cp, grep, tar, etc. fetchurl is a function that downloads files. perl is the Perl interpreter. Nix functions generally have the form {x, y, ..., z}: e where x, y, etc. are the names of the expected arguments, and where e 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 component. So we have to build a component. Building something from other stuff is called a derivation in Nix (as opposed to sources, which are built by humans instead of computers). We perform a derivation by calling stdenv.mkDerivation. mkDerivation is a function provided by stdenv that builds a component from a set of attributes. An attribute set is just a list of key/value pairs where each value is an arbitrary Nix expression. They take the general form {name1 = expr1; ... nameN = exprN;}. The attribute name specifies the symbolic name and version of the component. Nix doesn't really care about these things, but they are used by for instance nix-env -q to show a human-readable name for components. This attribute is required by mkDerivation. The attribute builder specifies the builder. This attribute can sometimes be omitted, in which case mkDerivation will fill in a default builder (which does a configure; make; make install, 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 ./builder.sh refers to the shell script shown in , discussed below. The builder has to know what the sources of the component are. Here, the attribute src is bound to the result of a call to the fetchurl 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. Instead of src any other name could have been used, and in fact there can be any number of sources (bound to different attributes). However, src is customary, and it's also expected by the default builder (which we don't use in this example). Since the derivation requires Perl, we have to pass the value of the perl function argument to the builder. All attributes in the set are actually passed as environment variables to the builder, so declaring an attribute perl = perl; will do the trick: it binds an attribute perl to the function argument which also happens to be called perl. However, it looks a bit silly, so there is a shorter syntax. The inherit keyword causes the specified attributes to be bound to whatever variables with the same name happen to be in scope.

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

Composition Composing GNU Hello (<filename>all-packages.nix</filename>) ... rec { hello = (import ../applications/misc/hello/ex-1 ) { inherit fetchurl stdenv perl; }; perl = (import ../development/interpreters/perl) { inherit fetchurl stdenv; }; fetchurl = (import ../build-support/fetchurl) { inherit stdenv; ... }; stdenv = ...; } The Nix expression in 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 pkgs/top-level/all-packages.nix, where all Nix expressions for components are imported and called with the appropriate arguments. shows some fragments of all-packages.nix. This file defines a set of attributes, all of which are concrete derivations (i.e., not functions). In fact, we define a mutually recursive set of attributes. That is, the attributes can refer to each other. This is precisely what we want since we want to plug the various components into each other. Here we import 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 in all-packages.nix at this point. That would be completely equivalent, but it would make the file rather bulky. Note that we refer to ../applications/misc/hello/ex-1, not ../applications/misc/hello/ex-1/default.nix. When you try to import a directory, Nix automatically appends /default.nix to the file name. This is where the actual composition takes place. Here we call the function imported from ../applications/misc/hello/ex-1 with an attribute set containing the things that the function expects, namely fetchurl, stdenv, and perl. We use inherit again to use the attributes defined in the surrounding scope (we could also have written fetchurl = fetchurl;, etc.). 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 stdenv.mkDerivation in ). Likewise, we have to instantiate Perl, fetchurl, and the standard environment.

Testing You can now try to build Hello. Of course, you could do nix-env -f pkgs/top-level/all-packages.nix -i hello, 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 nix-build, which builds a Nix expression and creates a symlink named result in the current directory: $ 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 ... $ ls -l result lrwxrwxrwx ... 2006-09-29 10:43 result -> /nix/store/632d2b22514d...-hello-2.1.1 $ ./result/bin/hello Hello, world! The option selects the hello attribute from all-packages.nix. This is faster than using the symbolic package name specified by the name attribute (which also happens to be hello) and is unambiguous (there can be multiple packages with the symbolic name hello, but there can be only one attribute in a set named hello). nix-build registers the ./result symlink as a garbage collection root, so unless and until you delete the ./result symlink, the output of the build will be safely kept on your system. You can use nix-build’s switch to give the symlink another name. 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 out is now valid. 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 path will not be registered as valid. If you try to build the derivation again, Nix will remove the output path if it exists (e.g., because the builder died half-way through make install) and try again. Note that there is no negative caching: 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). 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: $ nix-build pkgs/top-level/all-packages.nix -A hello waiting for lock on `/nix/store/0h5b7hp8d4hqfrw8igvx97x1xawrjnac-hello-2.1.1x' So it is always safe to run multiple instances of Nix in parallel (which isn’t the case with, say, make). 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 , where N is the maximum number of jobs to be run in parallel, or set. Typically this should be the number of CPUs.

The generic builder Recall from that the builder looked something like this: PATH=$perl/bin:$PATH tar xvfz $src cd hello-* ./configure --prefix=$out make make install 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 . Build script using the generic build functions buildInputs="$perl" source $stdenv/setup genericBuild The buildInputs variable tells setup to use the indicated components as inputs. This means that if a component provides a bin subdirectory, it's added to PATH; if it has a include subdirectory, it's added to GCC's header search path; and so on. How does it work? setup tries sourcing $out/nix-support/setup-hook of all dependencies thereby defining environment variables. If you have A depending on B depending on C you can force the setup-hooks of C beeing sourced when evaluating expression A by using propagatedBuildInputs=[ C ]; instead of buildInputs when defining derivation B. The function genericBuild is defined in the file $stdenv/setup. looks for a file nix-support/setup-hook within the derivation path. The final step calls the shell function genericBuild, which performs the steps that were done explicitly in . The generic builder is smart enough to figure out whether to unpack the sources using gzip, bzip2, etc. It can be customised in many ways; see . Discerning readers will note that the buildInputs could just as well have been set in the Nix expression, like this: buildInputs = [perl]; The perl attribute can then be removed, and the builder becomes even shorter: source $stdenv/setup genericBuild In fact, mkDerivation provides a default builder that looks exactly like that, so it is actually possible to omit the builder for Hello entirely.

The Nix expression language 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 normal values that can be passed around and manipulated in interesting ways. The language is not a full-featured, general purpose language. It's main job is to describe components, compositions of components, and the variability within components. This section presents the various features of the language.

Values Simple values Nix has the following basic datatypes: Strings are enclosed between double quotes, e.g., "foo bar". Strings can span multiple lines. The special characters " and \ and the character sequence ${ must be escaped by prefixing them with a backslash (\). Newlines, carriage returns and tabs can be written as \n, \r and \t, respectively. You can include the result of an expression into a string by enclosing it in ${...}, a feature known as antiquotation. The enclosed expression must evaluate to something that can be coerced into a string (meaning that it must be a string, a path, or a derivation). For instance, rather than writing "--with-freetype2-library=" + freetype + "/lib" (where freetype is a derivation), you can instead write the more natural "--with-freetype2-library=${freetype}/lib" The latter is automatically translated to the former. A more complicated example (from the Nix expression for Qt): 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"} "; 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., "-thread"), some of which in turn contain expressions (e.g., ${mesa}). As a convenience, URIs as defined in appendix B of RFC 2396 can be written as is, without quotes. For instance, the string "https://svn.cs.uu.nl:12443/dist/trace/trace-nix-trunk.tar.bz2" can also be written as https://svn.cs.uu.nl:12443/dist/trace/trace-nix-trunk.tar.bz2. Integers, e.g., 123. Paths, e.g., /bin/sh or ./builder.sh. A path must contain at least one slash to be recognised as such; for instance, builder.sh is not a pathIt's parsed as an expression that selects the attribute sh from the variable builder.. If the filename 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 /foo/bar/bla.nix refers to ../xyzzy/fnord.nix, the absolutised path is /foo/xyzzy/fnord.nix. Booleans with values true and false. Lists Lists are formed by enclosing a whitespace-separated list of values between square bracktes. For example, [ 123 ./foo.nix "abc" (f {x=y;}) ] defines a list of four elements, the last being the result of a call to the function f. Note that function calls have to be enclosed in parentheses. If they had been omitted, e.g., [ 123 ./foo.nix "abc" f {x=y;} ] the result would be a list of five elements, the fourth one being a function and the fifth being an attribute set. Attribute sets Attribute sets are really the core of the language, since ultimately it's all about creating derivations, which are really just sets of attributes to be passed to build scripts. Attribute sets are just a list of name/value pairs enclosed in curly brackets, where each value is an arbitrary expression terminated by a semicolon. For example: { x = 123; text = "Hello"; y = f { bla = 456; }; } This defines an attribute set with attributes named x, test, y. The order of the attributes is irrelevant. An attribute name may only occur once. Attributes can be selected from an attribute set using the . operator. For instance, { a = "Foo"; b = "Bar"; }.a evaluates to "Foo".

Language constructs Recursive attribute sets Recursive attribute sets are just normal attribute sets, but the attributes can refer to each other. For example, rec { x = y; y = 123; }.x evaluates to 123. Note that without rec the binding x = y; would refer to the variable y in the surrounding scope, if one exists, and would be invalid if no such variable exists. That is, in a normal (non-recursive) attribute set, attributes are not added to the lexical scope; in a recursive set, they are. Recursive attribute sets of course introduce the danger of infinite recursion. For example, rec { x = y; y = x; }.x does not terminateActually, Nix detects infinite recursion in this case and aborts (infinite recursion encountered).. Let-expressions A let-expression allows you define local variables for an expression. For instance, let x = "foo"; y = "bar"; in x + y evaluates to "foobar". There is also an obsolete form of let-expression, let { attrs }, which is translated to rec { attrs }.body. That is, the body of the let-expression is the body attribute of the attribute set. Inheriting attributes When defining an attribute 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 inherit keyword. For instance, let x = 123; in { inherit x; y = 456; } evaluates to {x = 123; y = 456;}. (Note that this works because x is added to the lexical scope by the let construct.) It is also possible to inherit attributes from another attribute set. For instance, in this fragment from all-packages.nix, graphviz = (import ../tools/graphics/graphviz) { inherit fetchurl stdenv libpng libjpeg expat x11 yacc; inherit (xlibs) libXaw; }; xlibs = { libX11 = ...; libXaw = ...; ... } libpng = ...; libjpg = ...; ... the attribute set used in the function call to the function defined in ../tools/graphics/graphviz inherits a number of variables from the surrounding scope (fetchurl ... yacc), but also inherits libXaw (the X Athena Widgets) from the xlibs (X11 client-side libraries) attribute set. Functions Functions have the following form: {params}: body This defines a function that must be called with an attribute set containing the attributes listed in params, which is a comma-separated list of attribute names. Optionally, for each parameter a default value may be specified by writing param ? e, where e is an arbitrary expression. If a parameter has a default, the corresponding attribute may be omitted in function calls. Note that functions do not have names. If you want to give them a name, you can bind them to an attribute, e.g., let concat = {x, y}: x + y; in concat {x = "foo"; y = "bar";} It is also possible to define a function that takes a single argument and that does not need to be called with an attribute set as argument. The syntax is var: body where var is the name of the argument. It is not possible to define a default. Example: let negate = x: !x; concat = x: y: x + y; in if negate true then concat "foo" "bar" else "" Note that concat is a function that takes one arguments 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., map (concat "foo") ["bar" "bla" "abc"] evaluates to ["foobar" "foobla" "fooabc"]. Conditionals Conditionals look like this: if e1 then e2 else e3 where e1 is an expression that should evaluate to a boolean value (true or false). Assertions Assertions are generally used to check that certain requirements on or between features and dependencies hold. They look like this: assert e1; e2 where e1 is an expression that should evaluate to a boolean value. If it evaluates to true, e2 is returned; otherwise expression evaluation is aborted and a backtrace is printed. Nix expression for Subversion { 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; assert httpServer -> httpd != null && httpd.expat == expat; assert sslSupport -> openssl != null && (httpServer -> httpd.openssl == openssl); 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; ... } show how assertions are used in the Nix expression for Subversion. 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 localServer argument set to true but the db4 argument set to null, then the evaluation fails. This is a more subtle condition: if Subversion is built with Apache (httpServer) 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. This assertion says that in order for Subversion to have SSL support (so that it can access https URLs), an OpenSSL library must be passed. Additionally, it says that if 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.) 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-null value was passed. This prevents an unnecessary rebuild of Subversion if OpenSSL changes. With-expressions A with-expression, with e1; e2 introduces the attribute set e1 into the lexical scope of the expression e2. For instance, let as = {x = "foo"; y = "bar";}; in with as; x + y evaluates to "foobar" since the with adds the x and y attributes of as to the lexical scope in the expression x + y. The most common use of with is in conjunction with the import function. E.g., with (import ./definitions.nix); ... makes all attributes defined in the file definitions.nix available as if they were defined locally in a rec-expression. Comments Comments can be single-line, started with a # character, or inline/multi-line, enclosed within /* ... */.

Operators lists the operators in the Nix expression language, in order of precedence (from strongest to weakest binding). Operators Syntax Associativity Description e . id none Select attribute named id from attribute set e. Abort evaluation if the attribute doesn’t exist. e1 e2 left Call function e1 with argument e2. e ? id none Test whether attribute set e contains an attribute named id; return true or false. e1 ++ e2 right List concatenation. e1 + e2 left String or path concatenation. ! e left Boolean negation. e1 // e2 right Return an attribute set consisting of the attributes in e1 and e2 (with the latter taking precedence over the former in case of equally named attributes). e1 == e2 none Equality. e1 != e2 none Inequality. e1 && e2 left Logical AND. e1 || e2 left Logical OR. e1 -> e2 none Logical implication (equivalent to !e1 || e2).

Derivations The most important built-in function is derivation, which is used to describe a single derivation (a build action). It takes as input an attribute set, the attributes of which specify the inputs of the build. There must be an attribute named system whose value must be a string specifying a Nix platform identifier, such as "i686-linux" or "powerpc-darwin"To figure out your platform identifier, look at the line Checking for the canonical Nix system name in the output of Nix's configure script. 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 .) There must be an attribute named name whose value must be a string. This is used as a symbolic name for the component by nix-env, and it is appended to the hash in the output path of the derivation. There must be an attribute named builder 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., ./builder.sh). Every attribute is passed as an environment variable to the builder. Attribute values are translated to environment variables as follows: Strings, URIs, and integers are just passed verbatim. A path (e.g., ../foo/sources.tar) 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. A derivation causes that derivation to be built prior to the present derivation; the output path is put in the environment variable. Lists of the previous types are also allowed. They are simply concatenated, separated by spaces. true is passed as 1, false and null are passed as empty string. The optional argument args specifies command-line arguments to be passed to the builder. It should be a list. (Note that mkDerivation in the standard environment is a wrapper around derivation that adds a default value for system and always uses Bash as the builder, to which the supplied builder is passed as a command-line argument. See .) The builder is executed as follows: A temporary directory is created under the directory specified by TMPDIR (default /tmp) where the build will take place. The current directory is changed to this directory. The environment is cleared and set to the derivation attributes, as specified above. In addition, the following variables are set: NIX_BUILD_TOP contains the path of the temporary directory for this build. Also, TMPDIR, TEMPDIR, TMP, TEMP 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. PATH is set to /path-not-set to prevent shells from initialising it to their built-in default value. HOME is set to /homeless-shelter to prevent programs from using /etc/passwd or the like to find the user's home directory, which could cause impurity. Usually, when HOME is set, it is used as the location of the home directory, even if it points to a non-existent path. NIX_STORE is set to the path of the top-level Nix store directory (typically, /nix/store). out is set to point to the output path of the derivation, which is a subdirectory of the Nix store. The output path is a concatenation of the cryptographic hash of all build inputs, and the name attribute. If the 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. A log of the combined standard output and error is written to /nix/var/log/nix. The builder is executed with the arguments specified by the attribute args. If it exits with exit code 0, it is considered to have succeeded. The temporary directory is removed (unless the option was specified). If the build was successful, Nix scans the output for references to the paths of the inputs. These so-called retained dependencies could be used when the output of the derivation is used (e.g., when it's executed or used as input to another derivation), so if we deploy the derivation, we should copy the retained dependencies as well. The scan is performed by looking for the hash parts of file names of the inputs. After the build, Nix sets the last-modified timestamp on all files in the build result to 0 (00:00:00 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 setuid and setgid 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.

Built-in functions This section lists the functions and constants built into the Nix expression evaluator. (The built-in function derivation is discussed above.) Some built-ins, such as derivation, 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 builtins built-in value, which is an attribute set that contains all built-in functions and values. For instance, derivation is also available as builtins.derivation. abort s Abort Nix expression evaluation, print error message s. builtins.add e1 e2 Return the sum of the integers e1 and e2. builtins.attrNames attrs Return the names of the attributes in the attribute set attrs in a sorted list. For instance, builtins.attrNames {y = 1; x = "foo";} evaluates to ["x" "y"]. There is no built-in function attrValues, but you can easily define it yourself: attrValues = attrs: map (name: builtins.getAttr name attrs) (builtins.attrNames attrs); baseNameOf s Return the base name of the string s, that is, everything following the final slash in the string. This is similar to the GNU basename command. builtins The attribute set builtins contains all the built-in functions and values. You can use builtins to test for the availability of features in the Nix installation, e.g., if builtins ? getEnv then builtins.getEnv "PATH" else "" 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 if builtins ? getEnv then __getEnv "PATH" else "" This Nix expression will trigger an “undefined variable” error on older Nix versions since __getEnv doesn’t exist. builtins.getEnv, on the other hand, is safe since builtins always exists and attribute selection is lazy, so it’s only performed if the test succeeds. builtins.currentSystem The built-in value currentSystem evaluates to the Nix platform identifier for the Nix installation on which the expression is being evaluated, such as "i686-linux" or "powerpc-darwin". derivation attrs derivation is described in . dirOf s Return the directory part of the string s, that is, everything before the final slash in the string. This is similar to the GNU dirname command. builtins.getAttr s attrs getAttr returns the attribute named s from the attribute set attrs. Evaluation aborts if the attribute doesn’t exist. This is a dynamic version of the . operator, since s is an expression rather than an identifier. builtins.getEnv s getEnv returns the value of the environment variable s, 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. getEnv is used in Nix Packages to locate the file ~/.nixpkgs/config.nix, which contains user-local settings for Nix Packages. (That is, it does a getEnv "HOME" to locate the user’s home directory.) builtins.hasAttr s attrs hasAttr returns true if the attribute set attrs has an attribute named s, and false otherwise. This is a dynamic version of the ? operator, since s is an expression rather than an identifier. builtins.head list 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 []. import path Load, parse and return the Nix expression in the file path. Evaluation aborts if the file doesn’t exist or contains an incorrect Nix expression. import 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. A Nix expression loaded by import must not contain any free variables (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 rec { x = 123; y = import ./foo.nix; } then the following foo.nix will give an error: x + 456 since x is not in scope in foo.nix. If you want x to be available in foo.nix, you should pass it as a function argument: rec { x = 123; y = import ./foo.nix x; } and x: x + 456 (The function argument doesn’t have to be called x in foo.nix; any name would work.) builtins.isList e Return true if e evaluates to a list, and false otherwise. isNull e Return true if e evaluates to null, and false otherwise. This function is deprecated; just write e == null instead. builtins.lessThan e1 e2 Return true if the integer e1 is less than the integer e2, and false otherwise. Evaluation aborts if either e1 or e2 does not evaluate to an integer. map f list Apply the function f to each element in the list list. For example, map (x: "foo" + x) ["bar" "bla" "abc"] evaluates to ["foobar" "foobla" "fooabc"]. builtins.pathExists path Return true if the path path exists, and false otherwise. One application of this function is to conditionally include a Nix expression containing user configuration: let fileName = builtins.getEnv "CONFIG_FILE"; config = if fileName != "" && builtins.pathExists (builtins.toPath fileName) then import (builtins.toPath fileName) else { someSetting = false; }; # default configuration in config.someSetting (Note that CONFIG_FILE must be an absolute path for this to work.) removeAttrs attrs list Remove the attributes listed in list from the attribute set attrs. The attributes don’t have to exist in attrs. For instance, removeAttrs { x = 1; y = 2; z = 3; } ["a" "x" "z"] evaluates to {y = 2;}. builtins.tail list Return the second to last elements of a list; abort evaluation if the argument isn’t a list or is an empty list. builtins.toFile name s Store the string s in a file in the Nix store and return its path. The file has suffix name. This file can be used as an input to derivations. One application is to write builders “inline”. For instance, the following Nix expression combines and into one file: {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; } It is even possible for one file to refer to another, e.g., builder = let configFile = builtins.toFile "foo.conf" " # This is some dummy configuration file. ... "; in builtins.toFile "builder.sh" " source $stdenv/setup ... cp ${configFile} $out/etc/foo.conf "; Note that ${configFile} is an antiquotation (see ), so the result of the expression configFile (i.e., a path like /nix/store/m7p7jfny445k...-foo.conf) will be spliced into the resulting string. It is however not allowed to have files mutually referring to each other, like so: let foo = builtins.toFile "foo" "...${bar}..."; bar = builtins.toFile "bar" "...${foo}..."; in foo This is not allowed because it would cause a cyclic dependency in the computation of the cryptographic hashes for foo and bar. builtins.toPath s Convert the string value s into a path value. The string s must represent an absolute path (i.e., must start with /). The path need not exist. The resulting path is canonicalised, e.g., builtins.toPath "//foo/xyzzy/../bar/" returns /foo/bar. toString e Convert the expression e to a string. e can be a string (in which case toString is a no-op) or a path (e.g., toString /foo/bar yields "/foo/bar". builtins.toXML e Return a string containing an XML representation of e. The main application for toXML is to communicate information with the builder in a more structured format than plain environment variables. shows an example where this is the case. The builder is supposed to generate the configuration file for a Jetty servlet container. A servlet container contains a number of servlets (*.war files) each exported under a specific URI prefix. So the servlet configuration is a list of attribute sets containing the path and war of the servlet (). 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 toXML, which is unambiguous and can easily be processed with the appropriate tools. For instance, in the example an XSLT stylesheet () is applied to it () to generate the XML configuration file for the Jetty server. The XML representation produced from by toXML is shown in . Note that uses the toFile 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 xsltproc ${stylesheet} .... Passing information to a builder using <function>toXML</function> $out/server-conf.xml]]>

"; servlets = builtins.toXML []]> XML representation produced by <function>toXML</function> ]]>

The standard environment The standard build environment in the Nix Packages collection provides a basic environment for building Unix packages. It consists of the following components: The GNU C Compiler, configured with C and C++ support. On Linux, the compiler has been patched to provide greater purity assurance. For instance, the compiler doesn't search in locations such as /usr/include. In fact, attempts to add such directories through the flag are filtered out. Likewise, the linker (from GNU binutils) doesn't search in standard locations such as /usr/lib. Programs built on Linux are linked against a GNU C Library that likewise doesn't search in the default system locations. GNU coreutils (contains a few dozen standard Unix commands). GNU findutils (contains find). GNU diffutils (contains diff, cmp). GNU sed. GNU grep. GNU awk. GNU tar. gzip and bzip2. GNU Make. It has been patched to provide nested output that can be fed into the nix-log2xml command and log2html stylesheet to create a structured, readable output of the build steps performed by Make. Bash. This is the shell used for all builders in the Nix Packages collection. Not using /bin/sh removes a large source of portability problems. Patch. The standard environment is used by passing it as an input called stdenv to the derivation, and then doing source $stdenv/setup at the top of the builder. Apart from adding the aforementioned commands to the PATH, setup also does the following: All input components specified in the buildInputs environment variable have their /bin subdirectory added to PATH, their /include subdirectory added to the C/C++ header file search path, and their /lib subdirectory added to the linker search path. This can be extended. For instance, when the pkgconfig component is used, the subdirectory /lib/pkgconfig of each input is added to the PKG_CONFIG_PATH environment variable. The environment variable NIX_CFLAGS_STRIP is set so that the compiler strips debug information from object files. This can be disabled by setting NIX_STRIP_DEBUG to 0. The setup script also exports a function called genericBuild that knows how to build typical Autoconf-style components. It can be customised to perform builds for any type of component. It is advisable to use genericBuild since it provides facilities that are almost always useful such as unpacking of sources, patching of sources, nested logging, etc. The definitive, up-to-date documentation of the generic builder is the source itself, which resides in pkgs/stdenv/generic/setup.sh.

Customising the generic builder The operation of the generic builder can be modified in many places by setting certain variables. These hook variables are typically set to the name of some shell function defined by you. For instance, to perform some additional steps after make install you would set the postInstall variable: postInstall=myPostInstall myPostInstall() { mkdir $out/share/extra cp extrafiles/* $out/share/extra } The generic builder has a number of phases, each of which can be override in its entirety by setting the indicated variable. The phases are: unpackPhase unpacks the source files listed in the src environment variable to the current directory. It supports tar files, optionally compressed with gzip or bzip2; Zip files (but note that the unzip command is not a part of the standard environment; you should add it as a build input yourself); and unpacked source trees (i.e., directories; they are copied verbatim). You can add support for other file types by setting the findUnpacker hook. This hook should set the variable unpackCmd to contain the command to be executed to unpack the file. After unpacking all source files, unpackPhase changes the current directory to the directory created by unpacking the sources. If there are multiple source directories, you should set sourceRoot to the name of the intended directory. It also calls the hook postUnpack after unpacking. patchPhase calls the patch command with the option for each patch file listed in the patches variable. configurePhase runs the script called configure in the current directory with a set to the output path. You can add additional flags through the configureFlags variable. If configure does not exist, nothing happens. Before and after running configure, the hooks preConfigure and postConfigure are called, respectively. buildPhase calls make. You can set flags for make through the makeFlags variable. Before and after running make, the hooks preBuild and postBuild are called, respectively. checkPhase calls make check, but only if the doCheck variable is set to 1. Additional flags can be set through the checkFlags variable. installPhase calls make install. Additional flags can be set through the installFlags variable. It also strips any static libraries in the output path of debug information unless dontStrip is set to 1. Before and after running make install, the hooks preInstall and postInstall are called, respectively. distPhase calls make dist, but only if the doDist variable is set to 1. Additional flags can be set through the distFlags variable. The resulting tarball is copied to the /tarballs subdirectory of the output path. Before and after running make dist, the hooks preDist and postDist are called, respectively. You can change the order in which phases are executed, or add new phases, by setting the phases variable. The default is patchPhase configurePhase buildPhase checkPhase installPhase distPhase.

Debugging failed builds At the beginning of each phase, the set of all shell variables is written to the file env-vars 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 flag, you can go to the output directory and switch to the environment of the builder: $ nix-build -K ./foo.nix ... fails, keeping build directory `/tmp/nix-1234-0' $ cd /tmp/nix-1234-0 $ source env-vars (edit some files...) $ make (execution continues with the same GCC, make, etc.)