This chapter shows you how to write Nix expressions, which are the things that tell Nix how to build components. It starts with a simple example (a Nix expression for GNU Hello), and then moves on to a more in-depth look at the Nix expression language. 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/system/all-packages-generic.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. {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. (See pkgs/stdenv/nix/path.nix to see what's in stdenv.) 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 the value is an arbitrary Nix expression. They take the general form {name1 = expr1; ... name1 = expr1;. 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 a MD5 hash of the expected contents of the file at that URL, this function actually 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 trink: 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. . $stdenv/setup PATH=$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. 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 -e option, which causes the script to be aborted if any command fails without an error check. ... 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/system/all-packages-generic.nix, where all Nix expressions for components are imported and called with the appropriate arguments. shows some fragments of all-packages-generic.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-generic.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. You can now try to build Hello. The simplest way to do that is by using nix-env: $ nix-env -f pkgs/system/i686-linux.nix -i hello installing `hello-2.1.1' building path `/nix/store/632d2b22514dcebe704887c3da15448d-hello-2.1.1' hello-2.1.1/ hello-2.1.1/intl/ hello-2.1.1/intl/ChangeLog ... This will build Hello and install it into your profile. The file i686-linux is just a simple Nix expression that imports all-packages-generic.nix and instantiates it for Linux on the x86 platform. Note that the hello argument here refers to the symbolic name given to the Hello derivation (the name attribute in ), not the hello attribute in all-packages-generic.nix. nix-env simply walks through all derivations defined in the latter file, looking for one with a name attribute matching the command-line argument. You can test whether it works: $ hello Hello, world! Generally, however, using nix-env is not the best way to test components, since you may not want to install them into your profile right away (they might not work properly, after all). A better way is to write a short file containging the following: (import pkgs/system/i686-linux.nix).hello Call it test.nix. Then you can build it without installing it using the command nix-build: $ nix-build ./test.nix ... /nix/store/632d2b22514dcebe704887c3da15448d-hello-2.1.1 nix-build will build the derivation and print the output path. It also creates a symlink to the output path called result in the current directory. This is convenient for testing the component: $ ./result/bin/hello Hello, world! Nix has a transactional semantics. Once a build finishes succesfully, 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. So it is always safe to run multiple instances of Nix in parallel (contrary to, 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 -j N, where N is the maximum number of jobs to be run in parallel. Typically this should be the number of CPUs.