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# Advanced Attributes
Derivations can declare some infrequently used optional attributes.
- `allowedReferences`
The optional attribute `allowedReferences` specifies a list of legal
references (dependencies) of the output of the builder. For example,
allowedReferences = [];
enforces that the output of a derivation cannot have any runtime
dependencies on its inputs. To allow an output to have a runtime
dependency on itself, use `"out"` as a list item. This is used in
NixOS to check that generated files such as initial ramdisks for
booting Linux don’t have accidental dependencies on other paths in
the Nix store.
- `allowedRequisites`
This attribute is similar to `allowedReferences`, but it specifies
the legal requisites of the whole closure, so all the dependencies
recursively. For example,
allowedRequisites = [ foobar ];
enforces that the output of a derivation cannot have any other
runtime dependency than `foobar`, and in addition it enforces that
`foobar` itself doesn't introduce any other dependency itself.
- `disallowedReferences`
The optional attribute `disallowedReferences` specifies a list of
illegal references (dependencies) of the output of the builder. For
example,
disallowedReferences = [ foo ];
enforces that the output of a derivation cannot have a direct
runtime dependencies on the derivation `foo`.
- `disallowedRequisites`
This attribute is similar to `disallowedReferences`, but it
specifies illegal requisites for the whole closure, so all the
dependencies recursively. For example,
disallowedRequisites = [ foobar ];
enforces that the output of a derivation cannot have any runtime
dependency on `foobar` or any other derivation depending recursively
on `foobar`.
- `exportReferencesGraph`
This attribute allows builders access to the references graph of
their inputs. The attribute is a list of inputs in the Nix store
whose references graph the builder needs to know. The value of this
attribute should be a list of pairs `[ name1
path1 name2
path2 ...
]`. The references graph of each pathN will be stored in a text file
nameN in the temporary build directory. The text files have the
format used by `nix-store
--register-validity` (with the deriver fields left empty). For
example, when the following derivation is built:
derivation {
...
exportReferencesGraph = [ "libfoo-graph" libfoo ];
};
the references graph of `libfoo` is placed in the file
`libfoo-graph` in the temporary build directory.
`exportReferencesGraph` is useful for builders that want to do
something with the closure of a store path. Examples include the
builders in NixOS that generate the initial ramdisk for booting
Linux (a `cpio` archive containing the closure of the boot script)
and the ISO-9660 image for the installation CD (which is populated
with a Nix store containing the closure of a bootable NixOS
configuration).
- `impureEnvVars`
This attribute allows you to specify a list of environment variables
that should be passed from the environment of the calling user to
the builder. Usually, the environment is cleared completely when the
builder is executed, but with this attribute you can allow specific
environment variables to be passed unmodified. For example,
`fetchurl` in Nixpkgs has the line
impureEnvVars = [ "http_proxy" "https_proxy" ... ];
to make it use the proxy server configuration specified by the user
in the environment variables http\_proxy and friends.
This attribute is only allowed in [fixed-output
derivations](#fixed-output-drvs), where impurities such as these are
okay since (the hash of) the output is known in advance. It is
ignored for all other derivations.
> **Warning**
>
> `impureEnvVars` implementation takes environment variables from
> the current builder process. When a daemon is building its
> environmental variables are used. Without the daemon, the
> environmental variables come from the environment of the
> `nix-build`.
- `outputHash`; `outputHashAlgo`; `outputHashMode`
These attributes declare that the derivation is a so-called
*fixed-output derivation*, which means that a cryptographic hash of
the output is already known in advance. When the build of a
fixed-output derivation finishes, Nix computes the cryptographic
hash of the output and compares it to the hash declared with these
attributes. If there is a mismatch, the build fails.
The rationale for fixed-output derivations is derivations such as
those produced by the `fetchurl` function. This function downloads a
file from a given URL. To ensure that the downloaded file has not
been modified, the caller must also specify a cryptographic hash of
the file. For example,
fetchurl {
url = "http://ftp.gnu.org/pub/gnu/hello/hello-2.1.1.tar.gz";
sha256 = "1md7jsfd8pa45z73bz1kszpp01yw6x5ljkjk2hx7wl800any6465";
}
It sometimes happens that the URL of the file changes, e.g., because
servers are reorganised or no longer available. We then must update
the call to `fetchurl`, e.g.,
fetchurl {
url = "ftp://ftp.nluug.nl/pub/gnu/hello/hello-2.1.1.tar.gz";
sha256 = "1md7jsfd8pa45z73bz1kszpp01yw6x5ljkjk2hx7wl800any6465";
}
If a `fetchurl` derivation was treated like a normal derivation, the
output paths of the derivation and *all derivations depending on it*
would change. For instance, if we were to change the URL of the
Glibc source distribution in Nixpkgs (a package on which almost all
other packages depend) massive rebuilds would be needed. This is
unfortunate for a change which we know cannot have a real effect as
it propagates upwards through the dependency graph.
For fixed-output derivations, on the other hand, the name of the
output path only depends on the `outputHash*` and `name` attributes,
while all other attributes are ignored for the purpose of computing
the output path. (The `name` attribute is included because it is
part of the path.)
As an example, here is the (simplified) Nix expression for
`fetchurl`:
{ stdenv, curl }: # The curl program is used for downloading.
{ url, sha256 }:
stdenv.mkDerivation {
name = baseNameOf (toString url);
builder = ./builder.sh;
buildInputs = [ curl ];
# This is a fixed-output derivation; the output must be a regular
# file with SHA256 hash sha256.
outputHashMode = "flat";
outputHashAlgo = "sha256";
outputHash = sha256;
inherit url;
}
The `outputHashAlgo` attribute specifies the hash algorithm used to
compute the hash. It can currently be `"sha1"`, `"sha256"` or
`"sha512"`.
The `outputHashMode` attribute determines how the hash is computed.
It must be one of the following two values:
- `"flat"`
The output must be a non-executable regular file. If it isn’t,
the build fails. The hash is simply computed over the contents
of that file (so it’s equal to what Unix commands like
`sha256sum` or `sha1sum` produce).
This is the default.
- `"recursive"`
The hash is computed over the NAR archive dump of the output
(i.e., the result of [`nix-store
--dump`](#refsec-nix-store-dump)). In this case, the output can
be anything, including a directory tree.
The `outputHash` attribute, finally, must be a string containing the
hash in either hexadecimal or base-32 notation. (See the [`nix-hash`
command](#sec-nix-hash) for information about converting to and from
base-32 notation.)
- `passAsFile`
A list of names of attributes that should be passed via files rather
than environment variables. For example, if you have
```
passAsFile = ["big"];
big = "a very long string";
```
then when the builder runs, the environment variable bigPath will
contain the absolute path to a temporary file containing `a very
long
string`. That is, for any attribute x listed in `passAsFile`, Nix
will pass an environment variable xPath holding the path of the file
containing the value of attribute x. This is useful when you need to
pass large strings to a builder, since most operating systems impose
a limit on the size of the environment (typically, a few hundred
kilobyte).
- `preferLocalBuild`
If this attribute is set to `true` and [distributed building is
enabled](#chap-distributed-builds), then, if possible, the derivaton
will be built locally instead of forwarded to a remote machine. This
is appropriate for trivial builders where the cost of doing a
download or remote build would exceed the cost of building locally.
- `allowSubstitutes`
If this attribute is set to `false`, then Nix will always build this
derivation; it will not try to substitute its outputs. This is
useful for very trivial derivations (such as `writeText` in Nixpkgs)
that are cheaper to build than to substitute from a binary cache.
> **Note**
>
> You need to have a builder configured which satisfies the
> derivation’s `system` attribute, since the derivation cannot be
> substituted. Thus it is usually a good idea to align `system` with
> `builtins.currentSystem` when setting `allowSubstitutes` to
> `false`. For most trivial derivations this should be the case.

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# Arguments and Variables
...
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 [???](#ex-hello-nix) is a function; it is missing
some arguments that have to be filled in somewhere. In the Nix Packages
collection this is done in the file `pkgs/top-level/all-packages.nix`,
where all Nix expressions for packages are imported and called with the
appropriate arguments. [example\_title](#ex-hello-composition) 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 packages 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 [???](#ex-hello-nix) 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 a 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 [???](#ex-hello-nix)).
> **Note**
>
> Nixpkgs has a convenience function `callPackage` that imports and
> calls a function, filling in any missing arguments by passing the
> corresponding attribute from the Nixpkgs set, like this:
>
> hello = callPackage ../applications/misc/hello/ex-1 { };
>
> If necessary, you can set or override arguments:
>
> hello = callPackage ../applications/misc/hello/ex-1 { stdenv = myStdenv; };
- Likewise, we have to instantiate Perl, `fetchurl`, and the standard
environment.

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# Build Script
source $stdenv/setup
PATH=$perl/bin:$PATH
tar xvfz $src
cd hello-*
./configure --prefix=$out
make
make install
[example\_title](#ex-hello-builder) 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 empty\[1\]. 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 [???](#ex-hello-nix), 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 package (since it was passed in as an attribute to the
derivation), so `$perl/bin` is the directory containing the Perl
interpreter.
- Now we have to unpack the sources. The `src` attribute was bound to
the result of fetching the Hello source tarball from the network, so
the src environment variable points to the location in the Nix store
to which the tarball was downloaded. After unpacking, we `cd` to the
resulting source directory.
The whole build is performed in a temporary directory created in
`/tmp`, by the way. This directory is removed after the builder
finishes, so there is no need to clean up the sources afterwards.
Also, the temporary directory is always newly created, so you don't
have to worry about files from previous builds interfering with the
current build.
- GNU Hello is a typical Autoconf-based package, so we first have to
run its `configure` script. In Nix every package is stored in a
separate location in the Nix store, for instance
`/nix/store/9a54ba97fb71b65fda531012d0443ce2-hello-2.1.1`. Nix
computes this path by cryptographically hashing all attributes of
the derivation. The path is passed to the builder through the out
environment variable. So here we give `configure` the parameter
`--prefix=$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.
1. Actually, it's initialised to `/path-not-set` to prevent Bash from
setting it to a default value.

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# Builder Syntax
source $stdenv/setup
PATH=$perl/bin:$PATH
tar xvfz $src
cd hello-*
./configure --prefix=$out
make
make install
[example\_title](#ex-hello-builder) 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 empty\[1\]. 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 [???](#ex-hello-nix), 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 package (since it was passed in as an attribute to the
derivation), so `$perl/bin` is the directory containing the Perl
interpreter.
- Now we have to unpack the sources. The `src` attribute was bound to
the result of fetching the Hello source tarball from the network, so
the src environment variable points to the location in the Nix store
to which the tarball was downloaded. After unpacking, we `cd` to the
resulting source directory.
The whole build is performed in a temporary directory created in
`/tmp`, by the way. This directory is removed after the builder
finishes, so there is no need to clean up the sources afterwards.
Also, the temporary directory is always newly created, so you don't
have to worry about files from previous builds interfering with the
current build.
- GNU Hello is a typical Autoconf-based package, so we first have to
run its `configure` script. In Nix every package is stored in a
separate location in the Nix store, for instance
`/nix/store/9a54ba97fb71b65fda531012d0443ce2-hello-2.1.1`. Nix
computes this path by cryptographically hashing all attributes of
the derivation. The path is passed to the builder through the out
environment variable. So here we give `configure` the parameter
`--prefix=$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.
1. Actually, it's initialised to `/path-not-set` to prevent Bash from
setting it to a default value.

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# 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 a set that contains all built-in functions and values.
For instance, `derivation` is also available as `builtins.derivation`.
- `abort` s; `builtins.abort` s
Abort Nix expression evaluation, print error message s.
- `builtins.add` e1 e2
Return the sum of the numbers e1 and e2.
- `builtins.all` pred list
Return `true` if the function pred returns `true` for all elements
of list, and `false` otherwise.
- `builtins.any` pred list
Return `true` if the function pred returns `true` for at least one
element of list, and `false` otherwise.
- `builtins.attrNames` set
Return the names of the attributes in the set set in an
alphabetically sorted list. For instance, `builtins.attrNames { y
= 1; x = "foo"; }` evaluates to `[ "x" "y" ]`.
- `builtins.attrValues` set
Return the values of the attributes in the set set in the order
corresponding to the sorted attribute names.
- `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.bitAnd` e1 e2
Return the bitwise AND of the integers e1 and e2.
- `builtins.bitOr` e1 e2
Return the bitwise OR of the integers e1 and e2.
- `builtins.bitXor` e1 e2
Return the bitwise XOR of the integers e1 and e2.
- `builtins`
The 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.
- `builtins.compareVersions` s1 s2
Compare two strings representing versions and return `-1` if version
s1 is older than version s2, `0` if they are the same, and `1` if s1
is newer than s2. The version comparison algorithm is the same as
the one used by [`nix-env
-u`](#ssec-version-comparisons).
- `builtins.concatLists` lists
Concatenate a list of lists into a single list.
- `builtins.concatStringsSep` separator list
Concatenate a list of strings with a separator between each element,
e.g. `concatStringsSep "/"
["usr" "local" "bin"] == "usr/local/bin"`
- `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 `"x86_64-darwin"`.
- `builtins.deepSeq` e1 e2
This is like `seq
e1
e2`, except that e1 is evaluated *deeply*: if it’s a list or set,
its elements or attributes are also evaluated recursively.
- `derivation` attrs; `builtins.derivation` attrs
`derivation` is described in [???](#ssec-derivation).
- `dirOf` s; `builtins.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.div` e1 e2
Return the quotient of the numbers e1 and e2.
- `builtins.elem` x xs
Return `true` if a value equal to x occurs in the list xs, and
`false` otherwise.
- `builtins.elemAt` xs n
Return element n from the list xs. Elements are counted starting
from 0. A fatal error occurs if the index is out of bounds.
- `builtins.fetchurl` url
Download the specified URL and return the path of the downloaded
file. This function is not available if [restricted evaluation
mode](#conf-restrict-eval) is enabled.
- `fetchTarball` url; `builtins.fetchTarball` url
Download the specified URL, unpack it and return the path of the
unpacked tree. The file must be a tape archive (`.tar`) compressed
with `gzip`, `bzip2` or `xz`. The top-level path component of the
files in the tarball is removed, so it is best if the tarball
contains a single directory at top level. The typical use of the
function is to obtain external Nix expression dependencies, such as
a particular version of Nixpkgs, e.g.
with import (fetchTarball https://github.com/NixOS/nixpkgs/archive/nixos-14.12.tar.gz) {};
stdenv.mkDerivation { … }
The fetched tarball is cached for a certain amount of time (1 hour
by default) in `~/.cache/nix/tarballs/`. You can change the cache
timeout either on the command line with `--option tarball-ttl number
of seconds` or in the Nix configuration file with this option: `
number of seconds to cache `.
Note that when obtaining the hash with ` nix-prefetch-url
` the option `--unpack` is required.
This function can also verify the contents against a hash. In that
case, the function takes a set instead of a URL. The set requires
the attribute `url` and the attribute `sha256`, e.g.
with import (fetchTarball {
url = "https://github.com/NixOS/nixpkgs/archive/nixos-14.12.tar.gz";
sha256 = "1jppksrfvbk5ypiqdz4cddxdl8z6zyzdb2srq8fcffr327ld5jj2";
}) {};
stdenv.mkDerivation { … }
This function is not available if [restricted evaluation
mode](#conf-restrict-eval) is enabled.
- `builtins.fetchGit` args
Fetch a path from git. args can be a URL, in which case the HEAD of
the repo at that URL is fetched. Otherwise, it can be an attribute
with the following attributes (all except `url` optional):
- url
The URL of the repo.
- name
The name of the directory the repo should be exported to in the
store. Defaults to the basename of the URL.
- rev
The git revision to fetch. Defaults to the tip of `ref`.
- ref
The git ref to look for the requested revision under. This is
often a branch or tag name. Defaults to `HEAD`.
By default, the `ref` value is prefixed with `refs/heads/`. As
of Nix 2.3.0 Nix will not prefix `refs/heads/` if `ref` starts
with `refs/`.
- submodules
A Boolean parameter that specifies whether submodules should be
checked out. Defaults to `false`.
<!-- end list -->
builtins.fetchGit {
url = "git@github.com:my-secret/repository.git";
ref = "master";
rev = "adab8b916a45068c044658c4158d81878f9ed1c3";
}
builtins.fetchGit {
url = "https://github.com/NixOS/nix.git";
ref = "refs/heads/0.5-release";
}
If the revision you're looking for is in the default branch of the
git repository you don't strictly need to specify the branch name in
the `ref` attribute.
However, if the revision you're looking for is in a future branch
for the non-default branch you will need to specify the the `ref`
attribute as well.
builtins.fetchGit {
url = "https://github.com/nixos/nix.git";
rev = "841fcbd04755c7a2865c51c1e2d3b045976b7452";
ref = "1.11-maintenance";
}
> **Note**
>
> It is nice to always specify the branch which a revision belongs
> to. Without the branch being specified, the fetcher might fail if
> the default branch changes. Additionally, it can be confusing to
> try a commit from a non-default branch and see the fetch fail. If
> the branch is specified the fault is much more obvious.
If the revision you're looking for is in the default branch of the
git repository you may omit the `ref` attribute.
builtins.fetchGit {
url = "https://github.com/nixos/nix.git";
rev = "841fcbd04755c7a2865c51c1e2d3b045976b7452";
}
builtins.fetchGit {
url = "https://github.com/nixos/nix.git";
ref = "refs/tags/1.9";
}
`builtins.fetchGit` can behave impurely fetch the latest version of
a remote branch.
> **Note**
>
> Nix will refetch the branch in accordance to
> [???](#conf-tarball-ttl).
> **Note**
>
> This behavior is disabled in *Pure evaluation mode*.
builtins.fetchGit {
url = "ssh://git@github.com/nixos/nix.git";
ref = "master";
}
- `builtins.filter` f xs
Return a list consisting of the elements of xs for which the
function f returns `true`.
- `builtins.filterSource` e1 e2
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 `source-dir` as an input to a Nix expression, e.g.
stdenv.mkDerivation {
...
src = ./source-dir;
}
However, if `source-dir` is a Subversion working copy, then all
those annoying `.svn` subdirectories will also be copied to the
store. Worse, the contents of those directories may change a lot,
causing lots of spurious rebuilds. With `filterSource` you can
filter out the `.svn` directories:
```
src = builtins.filterSource
(path: type: type != "directory" || baseNameOf path != ".svn")
./source-dir;
```
Thus, the first argument e1 must be a predicate function that is
called for each regular file, directory or symlink in the source
tree e2. If the function returns `true`, 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
`"regular"`, `"directory"`, `"symlink"` or `"unknown"` (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
`true` for them, the copy will fail). If you exclude a directory,
the entire corresponding subtree of e2 will be excluded.
- `builtins.foldl’` op nul list
Reduce a list by applying a binary operator, from left to right,
e.g. `foldl’ op nul [x0 x1 x2 ...] = op (op
(op nul x0) x1) x2) ...`. The operator is applied strictly, i.e.,
its arguments are evaluated first. For example, `foldl’ (x: y: x +
y) 0 [1 2 3]` evaluates to 6.
- `builtins.functionArgs` f
Return a set containing the names of the formal arguments expected
by the function f. The value of each attribute is a Boolean denoting
whether the corresponding argument has a default value. For
instance, `functionArgs ({ x, y ? 123}: ...) = { x = false; y =
true; }`.
"Formal argument" here refers to the attributes pattern-matched by
the function. Plain lambdas are not included, e.g. `functionArgs (x:
...) = { }`.
- `builtins.fromJSON` e
Convert a JSON string to a Nix value. For example,
builtins.fromJSON ''{"x": [1, 2, 3], "y": null}''
returns the value `{ x = [ 1 2 3 ]; y = null;
}`.
- `builtins.genList` generator length
Generate list of size length, with each element i equal to the value
returned by generator `i`. For example,
builtins.genList (x: x * x) 5
returns the list `[ 0 1 4 9 16 ]`.
- `builtins.getAttr` s set
`getAttr` returns the attribute named s from set. 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 set
`hasAttr` returns `true` if set 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.hashString` type s
Return a base-16 representation of the cryptographic hash of string
s. The hash algorithm specified by type must be one of `"md5"`,
`"sha1"`, `"sha256"` or `"sha512"`.
- `builtins.hashFile` type p
Return a base-16 representation of the cryptographic hash of the
file at path p. The hash algorithm specified by type must be one of
`"md5"`, `"sha1"`, `"sha256"` or `"sha512"`.
- `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; `builtins.import` path
Load, parse and return the Nix expression in the file path. If path
is a directory, the file ` default.nix
` in that directory is loaded. 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 a set or a function) in a separate file, and use it from Nix
expressions in other files.
> **Note**
>
> Unlike some languages, `import` is a regular function in Nix.
> Paths using the angle bracket syntax (e.g., `
> > > > > import` \<foo\>) are normal path values (see [???](#ssec-values)).
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.intersectAttrs` e1 e2
Return a set consisting of the attributes in the set e2 that also
exist in the set e1.
- `builtins.isAttrs` e
Return `true` if e evaluates to a set, and `false` otherwise.
- `builtins.isList` e
Return `true` if e evaluates to a list, and `false` otherwise.
- `builtins.isFunction` e
Return `true` if e evaluates to a function, and `false` otherwise.
- `builtins.isString` e
Return `true` if e evaluates to a string, and `false` otherwise.
- `builtins.isInt` e
Return `true` if e evaluates to an int, and `false` otherwise.
- `builtins.isFloat` e
Return `true` if e evaluates to a float, and `false` otherwise.
- `builtins.isBool` e
Return `true` if e evaluates to a bool, and `false` otherwise.
- `builtins.isPath` e
Return `true` if e evaluates to a path, and `false` otherwise.
- `isNull` e; `builtins.isNull` e
Return `true` if e evaluates to `null`, and `false` otherwise.
> **Warning**
>
> This function is *deprecated*; just write `e == null` instead.
- `builtins.length` e
Return the length of the list e.
- `builtins.lessThan` e1 e2
Return `true` if the number e1 is less than the number e2, and
`false` otherwise. Evaluation aborts if either e1 or e2 does not
evaluate to a number.
- `builtins.listToAttrs` e
Construct a set from a list specifying the names and values of each
attribute. Each element of the list should be a set consisting of a
string-valued attribute `name` specifying the name of the attribute,
and an attribute `value` specifying its value. Example:
builtins.listToAttrs
[ { name = "foo"; value = 123; }
{ name = "bar"; value = 456; }
]
evaluates to
{ foo = 123; bar = 456; }
- `map` f list; `builtins.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.match` regex str
Returns a list if the [extended POSIX regular
expression](http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html#tag_09_04)
regex matches str precisely, otherwise returns `null`. Each item in
the list is a regex group.
builtins.match "ab" "abc"
Evaluates to `null`.
builtins.match "abc" "abc"
Evaluates to `[ ]`.
builtins.match "a(b)(c)" "abc"
Evaluates to `[ "b" "c" ]`.
builtins.match "[[:space:]]+([[:upper:]]+)[[:space:]]+" " FOO "
Evaluates to `[ "foo" ]`.
- `builtins.mul` e1 e2
Return the product of the numbers e1 and e2.
- `builtins.parseDrvName` s
Split the string s into a package name and version. The package name
is everything up to but not including the first dash followed by a
digit, and the version is everything following that dash. The result
is returned in a set `{ name, version }`. Thus,
`builtins.parseDrvName "nix-0.12pre12876"` returns `{ name = "nix";
version = "0.12pre12876";
}`.
- `builtins.path` args
An enrichment of the built-in path type, based on the attributes
present in args. All are optional except `path`:
- path
The underlying path.
- name
The name of the path when added to the store. This can used to
reference paths that have nix-illegal characters in their names,
like `@`.
- filter
A function of the type expected by
[builtins.filterSource](#builtin-filterSource), with the same
semantics.
- recursive
When `false`, when `path` is added to the store it is with a
flat hash, rather than a hash of the NAR serialization of the
file. Thus, `path` must refer to a regular file, not a
directory. This allows similar behavior to `fetchurl`. Defaults
to `true`.
- sha256
When provided, this is the expected hash of the file at the
path. Evaluation will fail if the hash is incorrect, and
providing a hash allows `builtins.path` to be used even when the
`pure-eval` nix config option is on.
- `builtins.pathExists` path
Return `true` if the path path exists at evaluation time, and
`false` otherwise.
- `builtins.placeholder` output
Return a placeholder string for the specified output that will be
substituted by the corresponding output path at build time. Typical
outputs would be `"out"`, `"bin"` or `"dev"`.
- `builtins.readDir` path
Return the contents of the directory path as a set mapping directory
entries to the corresponding file type. For instance, if directory
`A` contains a regular file `B` and another directory `C`, then
`builtins.readDir
./A` will return the set
{ B = "regular"; C = "directory"; }
The possible values for the file type are `"regular"`,
`"directory"`, `"symlink"` and `"unknown"`.
- `builtins.readFile` path
Return the contents of the file path as a string.
- `removeAttrs` set list; `builtins.removeAttrs` set list
Remove the attributes listed in list from set. The attributes don’t
have to exist in set. For instance,
removeAttrs { x = 1; y = 2; z = 3; } [ "a" "x" "z" ]
evaluates to `{ y = 2; }`.
- `builtins.replaceStrings` from to s
Given string s, replace every occurrence of the strings in from with
the corresponding string in to. For example,
builtins.replaceStrings ["oo" "a"] ["a" "i"] "foobar"
evaluates to `"fabir"`.
- `builtins.seq` e1 e2
Evaluate e1, then evaluate and return e2. This ensures that a
computation is strict in the value of e1.
- `builtins.sort` comparator list
Return list in sorted order. It repeatedly calls the function
comparator with two elements. The comparator should return `true` if
the first element is less than the second, and `false` otherwise.
For example,
builtins.sort builtins.lessThan [ 483 249 526 147 42 77 ]
produces the list `[ 42 77 147 249 483 526
]`.
This is a stable sort: it preserves the relative order of elements
deemed equal by the comparator.
- `builtins.split` regex str
Returns a list composed of non matched strings interleaved with the
lists of the [extended POSIX regular
expression](http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html#tag_09_04)
regex matches of str. Each item in the lists of matched sequences is
a regex group.
builtins.split "(a)b" "abc"
Evaluates to `[ "" [ "a" ] "c" ]`.
builtins.split "([ac])" "abc"
Evaluates to `[ "" [ "a" ] "b" [ "c" ] "" ]`.
builtins.split "(a)|(c)" "abc"
Evaluates to `[ "" [ "a" null ] "b" [ null "c" ] "" ]`.
builtins.split "([[:upper:]]+)" " FOO "
Evaluates to `[ " " [ "FOO" ] " " ]`.
- `builtins.splitVersion` s
Split a string representing a version into its components, by the
same version splitting logic underlying the version comparison in
[`nix-env -u`](#ssec-version-comparisons).
- `builtins.stringLength` e
Return the length of the string e. If e is not a string, evaluation
is aborted.
- `builtins.sub` e1 e2
Return the difference between the numbers e1 and e2.
- `builtins.substring` start len s
Return the substring of s from character position start (zero-based)
up to but not including start + len. If start is greater than the
length of the string, an empty string is returned, and if start +
len lies beyond the end of the string, only the substring up to the
end of the string is returned. start must be non-negative. For
example,
builtins.substring 0 3 "nixos"
evaluates to `"nix"`.
- `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.
- `throw` s; `builtins.throw` s
Throw an error message s. This usually aborts Nix expression
evaluation, but in `nix-env -qa` 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 `abort`).
- `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 [???](#ex-hello-nix)
and [???](#ex-hello-builder) 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://ftp.nluug.nl/pub/gnu/hello/hello-2.1.1.tar.gz";
sha256 = "1md7jsfd8pa45z73bz1kszpp01yw6x5ljkjk2hx7wl800any6465";
};
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
[???](#ssec-values)), 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`.
It is also not possible to reference the result of a derivation. If
you are using Nixpkgs, the `writeTextFile` function is able to do
that.
- `builtins.toJSON` e
Return a string containing a JSON representation of e. Strings,
integers, floats, booleans, nulls and lists are mapped to their JSON
equivalents. Sets (except derivations) are represented as objects.
Derivations are translated to a JSON string containing the
derivation’s output path. Paths are copied to the store and
represented as a JSON string of the resulting store path.
- `builtins.toPath` s
DEPRECATED. Use `/. + "/path"` to convert a string into an absolute
path. For relative paths, use `./. + "/path"`.
- `toString` e; `builtins.toString` e
Convert the expression e to a string. e can be:
- A string (in which case the string is returned unmodified).
- A path (e.g., `toString /foo/bar` yields `"/foo/bar"`.
- A set containing `{ __toString = self: ...; }`.
- An integer.
- A list, in which case the string representations of its elements
are joined with spaces.
- A Boolean (`false` yields `""`, `true` yields `"1"`).
- `null`, which yields the empty string.
- `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.
[example\_title](#ex-toxml) shows an example where this is the case.
The builder is supposed to generate the configuration file for a
[Jetty servlet container](http://jetty.mortbay.org/). 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 sets containing the `path` and `war` of the servlet
([co\_title](#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 `toXML`, which is unambiguous and can easily be
processed with the appropriate tools. For instance, in the example
an XSLT stylesheet ([co\_title](#ex-toxml-co-stylesheet)) is applied
to it ([co\_title](#ex-toxml-co-apply)) to generate the XML
configuration file for the Jetty server. The XML representation
produced from [co\_title](#ex-toxml-co-servlets) by `toXML` is shown
in [example\_title](#ex-toxml-result).
Note that [example\_title](#ex-toxml) 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}
...`.
{ 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
";
stylesheet = builtins.toFile "stylesheet.xsl"
"<?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 [
{ path = "/bugtracker"; war = jira + "/lib/atlassian-jira.war"; }
{ path = "/wiki"; war = uberwiki + "/uberwiki.war"; }
];
})
<?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>
- `builtins.trace` e1 e2
Evaluate e1 and print its abstract syntax representation on standard
error. Then return e2. This function is useful for debugging.
- `builtins.tryEval` e
Try to shallowly evaluate e. Return a set containing the attributes
`success` (`true` if e evaluated successfully, `false` if an error
was thrown) and `value`, equalling e if successful and `false`
otherwise. Note that this doesn't evaluate e deeply, so ` let e = {
x = throw ""; }; in (builtins.tryEval e).success
` will be `true`. Using ` builtins.deepSeq
` one can get the expected result: `let e = { x = throw "";
}; in (builtins.tryEval (builtins.deepSeq e e)).success` will be
`false`.
- `builtins.typeOf` e
Return a string representing the type of the value e, namely
`"int"`, `"bool"`, `"string"`, `"path"`, `"null"`, `"set"`,
`"list"`, `"lambda"` or `"float"`.

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# Derivations
The most important built-in function is `derivation`, which is used to
describe a single derivation (a build action). It takes as input a set,
the attributes of which specify the inputs of the build.
- There must be an attribute named `system` whose value must be a
string specifying a Nix platform identifier, such as `"i686-linux"`
or `"x86_64-darwin"`\[1\] 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 [???](#chap-distributed-builds).)
- There must be an attribute named `name` whose value must be a
string. This is used as a symbolic name for the package by
`nix-env`, and it is appended to the output paths 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 and numbers 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; its default 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 the string `1`, `false` and `null` are
passed as an empty string.
- The optional attribute `args` specifies command-line arguments to be
passed to the builder. It should be a list.
- The optional attribute `outputs` specifies a list of symbolic
outputs of the derivation. By default, a derivation produces a
single output path, denoted as `out`. However, derivations can
produce multiple output paths. This is useful because it allows
outputs to be downloaded or garbage-collected separately. For
instance, imagine a library package that provides a dynamic library,
header files, and documentation. A program that links against the
library doesn’t need the header files and documentation at runtime,
and it doesn’t need the documentation at build time. Thus, the
library package could specify:
outputs = [ "lib" "headers" "doc" ];
This will cause Nix to pass environment variables `lib`, `headers`
and `doc` to the builder containing the intended store paths of each
output. The builder would typically do something like
./configure --libdir=$lib/lib --includedir=$headers/include --docdir=$doc/share/doc
for an Autoconf-style package. You can refer to each output of a
derivation by selecting it as an attribute, e.g.
buildInputs = [ pkg.lib pkg.headers ];
The first element of `outputs` determines the *default output*.
Thus, you could also write
buildInputs = [ pkg pkg.headers ];
since `pkg` is equivalent to `pkg.lib`.
The function `mkDerivation` in the Nixpkgs 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 Nixpkgs manual for details.
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`).
- For each output declared in `outputs`, the corresponding
environment variable is set to point to the intended path in the
Nix store for that output. Each output path is a concatenation
of the cryptographic hash of all build inputs, the `name`
attribute and the output name. (The output name is omitted if
it’s `out`.)
- If an output path already exists, it is removed. Also, locks are
acquired to prevent multiple Nix instances from performing the same
build at the same time.
- 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 `-K` option was
specified).
- If the build was successful, Nix scans each output path for
references to input paths by looking for the hash parts of the input
paths. Since these are potential runtime dependencies, Nix registers
them as dependencies of the output paths.
- After the build, Nix sets the last-modified timestamp on all files
in the build result to 1 (00:00:01 1/1/1970 UTC), sets the group to
the default group, and sets the mode of the file to 0444 or 0555
(i.e., read-only, with execute permission enabled if the file was
originally executable). Note that possible `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.
<!-- end list -->
1. 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.

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# 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. Its main job is to describe
packages, compositions of packages, and the variability within packages.
This section presents the various features of the language.

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# Expression Syntax
{ 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";
sha256 = "1md7jsfd8pa45z73bz1kszpp01yw6x5ljkjk2hx7wl800any6465";
};
inherit perl;
}
[example\_title](#ex-hello-nix) 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 package that
is used by almost all Nix Packages packages; 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 package.
- So we have to build a package. 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 package from a set of *attributes*. A set
is just a list of key/value pairs where each key is a string and
each value is an arbitrary Nix expression. They take the general
form `{
name1 =
expr1; ...
nameN =
exprN; }`.
- The attribute `name` specifies the symbolic name and version of the
package. 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 packages. 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 [???](#ex-hello-builder), discussed below.
- The builder has to know what the sources of the package are. Here,
the attribute `src` is bound to the result of a call to the
`fetchurl` function. Given a URL and a SHA-256 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.

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# Generic Builder Syntax
Recall from [???](#ex-hello-builder) 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 [example\_title](#ex-hello-builder2).
buildInputs="$perl"
source $stdenv/setup
genericBuild
- The buildInputs variable tells `setup` to use the indicated packages
as “inputs”. This means that if a package 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.\[1\]
- The function `genericBuild` is defined in the file `$stdenv/setup`.
- The final step calls the shell function `genericBuild`, which
performs the steps that were done explicitly in
[???](#ex-hello-builder). 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 the Nixpkgs manual for
details.
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.
1. How does it work? `setup` tries to source the file
`pkg/nix-support/setup-hook` of all dependencies. These “setup
hooks” can then set up whatever environment variables they want;
for instance, the setup hook for Perl sets the PERL5LIB environment
variable to contain the `lib/site_perl` directories of all inputs.

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# Language Constructs
Recursive sets are just normal 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) set, attributes are not added to the lexical scope; in a
recursive set, they are.
Recursive sets of course introduce the danger of infinite recursion. For
example,
rec {
x = y;
y = x;
}.x
does not terminate\[1\].
A let-expression allows you to define local variables for an expression.
For instance,
let
x = "foo";
y = "bar";
in x + y
evaluates to `"foobar"`.
When defining a set or in a let-expression 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;
}
is equivalent to
let x = 123; in
{ x = x;
y = 456;
}
and both evaluate 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 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 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) set.
Summarizing the fragment
...
inherit x y z;
inherit (src-set) a b c;
...
is equivalent to
...
x = x; y = y; z = z;
a = src-set.a; b = src-set.b; c = src-set.c;
...
when used while defining local variables in a let-expression or while
defining a set.
Functions have the following form:
pattern: body
The pattern specifies what the argument of the function must look like,
and binds variables in the body to (parts of) the argument. There are
three kinds of patterns:
- If a pattern is a single identifier, then the function matches any
argument. 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 argument and returns
a function that takes another argument. This allows partial
parameterisation (i.e., only filling some of the arguments of a
function); e.g.,
map (concat "foo") [ "bar" "bla" "abc" ]
evaluates to `[ "foobar" "foobla"
"fooabc" ]`.
- A *set pattern* of the form `{ name1, name2, …, nameN }` matches a
set containing the listed attributes, and binds the values of those
attributes to variables in the function body. For example, the
function
{ x, y, z }: z + y + x
can only be called with a set containing exactly the attributes `x`,
`y` and `z`. No other attributes are allowed. If you want to allow
additional arguments, you can use an ellipsis (`...`):
{ x, y, z, ... }: z + y + x
This works on any set that contains at least the three named
attributes.
It is possible to provide *default values* for attributes, in which
case they are allowed to be missing. A default value is specified by
writing `name ?
e`, where e is an arbitrary expression. For example,
{ x, y ? "foo", z ? "bar" }: z + y + x
specifies a function that only requires an attribute named `x`, but
optionally accepts `y` and `z`.
- An `@`-pattern provides a means of referring to the whole value
being matched:
```
args@{ x, y, z, ... }: z + y + x + args.a
```
but can also be written as:
```
{ x, y, z, ... } @ args: z + y + x + args.a
```
Here `args` is bound to the entire argument, which is further
matched against the pattern `{ x, y, z,
... }`. `@`-pattern makes mainly sense with an ellipsis(`...`) as
you can access attribute names as `a`, using `args.a`, which was
given as an additional attribute to the function.
> **Warning**
>
> The `args@` expression is bound to the argument passed to the
> function which means that attributes with defaults that aren't
> explicitly specified in the function call won't cause an
> evaluation error, but won't exist in `args`.
>
> For instance
>
> let
> function = args@{ a ? 23, ... }: args;
> in
> function {}
>
> will evaluate to an empty attribute set.
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"; }
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 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.
{ 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;
...
}
[example\_title](#ex-subversion-nix) 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.
A *with-expression*,
with e1; e2
introduces the 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 `let`-expression.
The bindings introduced by `with` do not shadow bindings introduced by
other means, e.g.
let a = 3; in with { a = 1; }; let a = 4; in with { a = 2; }; ...
establishes the same scope as
let a = 1; in let a = 2; in let a = 3; in let a = 4; in ...
Comments can be single-line, started with a `#` character, or
inline/multi-line, enclosed within `/*
... */`.
1. Actually, Nix detects infinite recursion in this case and aborts
(“infinite recursion encountered”).

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# Operators
[table\_title](#table-operators) lists the operators in the Nix
expression language, in order of precedence (from strongest to weakest
binding).
| Name | Syntax | Associativity | Description | Precedence |
| ------------------------ | ----------------------------- | ------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ---------- |
| Select | e `.` attrpath \[ `or` def \] | none | Select attribute denoted by the attribute path attrpath from set e. (An attribute path is a dot-separated list of attribute names.) If the attribute doesn’t exist, return def if provided, otherwise abort evaluation. | 1 |
| Application | e1 e2 | left | Call function e1 with argument e2. | 2 |
| Arithmetic Negation | `-` e | none | Arithmetic negation. | 3 |
| Has Attribute | e `?` attrpath | none | Test whether set e contains the attribute denoted by attrpath; return `true` or `false`. | 4 |
| List Concatenation | e1 `++` e2 | right | List concatenation. | 5 |
| Multiplication | e1 `*` e2, | left | Arithmetic multiplication. | 6 |
| Division | e1 `/` e2 | left | Arithmetic division. | 6 |
| Addition | e1 `+` e2 | left | Arithmetic addition. | 7 |
| Subtraction | e1 `-` e2 | left | Arithmetic subtraction. | 7 |
| String Concatenation | string1 `+` string2 | left | String concatenation. | 7 |
| Not | `!` e | none | Boolean negation. | 8 |
| Update | e1 `//` e2 | right | Return a set consisting of the attributes in e1 and e2 (with the latter taking precedence over the former in case of equally named attributes). | 9 |
| Less Than | e1 `<` e2, | none | Arithmetic comparison. | 10 |
| Less Than or Equal To | e1 `<=` e2 | none | Arithmetic comparison. | 10 |
| Greater Than | e1 `>` e2 | none | Arithmetic comparison. | 10 |
| Greater Than or Equal To | e1 `>=` e2 | none | Arithmetic comparison. | 10 |
| Equality | e1 `==` e2 | none | Equality. | 11 |
| Inequality | e1 `!=` e2 | none | Inequality. | 11 |
| Logical AND | e1 `&&` e2 | left | Logical AND. | 12 |
| Logical OR | e1 `\|\|` e2 | left | Logical OR. | 13 |
| Logical Implication | e1 `->` e2 | none | Logical implication (equivalent to `!e1 \|\|
e2`). | 14 |
Operators

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# Values
Nix has the following basic data types:
- *Strings* can be written in three ways.
The most common way is to enclose the string 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](http://www.trolltech.com/products/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}`).
The second way to write string literals is as an *indented string*,
which is enclosed between pairs of *double single-quotes*, like so:
''
This is the first line.
This is the second line.
This is the third line.
''
This kind of string literal intelligently strips indentation from
the start of each line. To be precise, it strips from each line a
number of spaces equal to the minimal indentation of the string as a
whole (disregarding the indentation of empty lines). For instance,
the first and second line are indented two space, while the third
line is indented four spaces. Thus, two spaces are stripped from
each line, so the resulting string is
"This is the first line.\nThis is the second line.\n This is the third line.\n"
Note that the whitespace and newline following the opening `''` is
ignored if there is no non-whitespace text on the initial line.
Antiquotation (`${expr}`) is supported in indented strings.
Since `${` and `''` have special meaning in indented strings, you
need a way to quote them. `$` can be escaped by prefixing it with
`''` (that is, two single quotes), i.e., `''$`. `''` can be escaped
by prefixing it with `'`, i.e., `'''`. `$` removes any special
meaning from the following `$`. Linefeed, carriage-return and tab
characters can be written as `''\n`, `''\r`, `''\t`, and `''\`
escapes any other character.
Indented strings are primarily useful in that they allow multi-line
string literals to follow the indentation of the enclosing Nix
expression, and that less escaping is typically necessary for
strings representing languages such as shell scripts and
configuration files because `''` is much less common than `"`.
Example:
stdenv.mkDerivation {
...
postInstall =
''
mkdir $out/bin $out/etc
cp foo $out/bin
echo "Hello World" > $out/etc/foo.conf
${if enableBar then "cp bar $out/bin" else ""}
'';
...
}
Finally, as a convenience, *URIs* as defined in appendix B of
[RFC 2396](http://www.ietf.org/rfc/rfc2396.txt) can be written *as
is*, without quotes. For instance, the string
`"http://example.org/foo.tar.bz2"` can also be written as
`http://example.org/foo.tar.bz2`.
- Numbers, which can be *integers* (like `123`) or *floating point*
(like `123.43` or `.27e13`).
Numbers are type-compatible: pure integer operations will always
return integers, whereas any operation involving at least one
floating point number will have a floating point number as a result.
- *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 path\[1\]. If the file name is relative, i.e., if it does
not begin with a slash, it is made absolute at parse time relative
to the directory of the Nix expression that contained it. For
instance, if a Nix expression in `/foo/bar/bla.nix` refers to
`../xyzzy/fnord.nix`, the absolute path is `/foo/xyzzy/fnord.nix`.
If the first component of a path is a `~`, it is interpreted as if
the rest of the path were relative to the user's home directory.
e.g. `~/foo` would be equivalent to `/home/edolstra/foo` for a user
whose home directory is `/home/edolstra`.
Paths can also be specified between angle brackets, e.g.
`<nixpkgs>`. This means that the directories listed in the
environment variable NIX\_PATH will be searched for the given file
or directory name.
- *Booleans* with values `true` and `false`.
- The null value, denoted as `null`.
Lists are formed by enclosing a whitespace-separated list of values
between square brackets. 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 a set.
Note that lists are only lazy in values, and they are strict in length.
Sets are really the core of the language, since ultimately the Nix
language is all about creating derivations, which are really just sets
of attributes to be passed to build scripts.
Sets are just a list of name/value pairs (called *attributes*) 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 a set with attributes named `x`, `text`, `y`. The order of
the attributes is irrelevant. An attribute name may only occur once.
Attributes can be selected from a set using the `.` operator. For
instance,
{ a = "Foo"; b = "Bar"; }.a
evaluates to `"Foo"`. It is possible to provide a default value in an
attribute selection using the `or` keyword. For example,
{ a = "Foo"; b = "Bar"; }.c or "Xyzzy"
will evaluate to `"Xyzzy"` because there is no `c` attribute in the set.
You can use arbitrary double-quoted strings as attribute names:
{ "foo ${bar}" = 123; "nix-1.0" = 456; }."foo ${bar}"
This will evaluate to `123` (Assuming `bar` is antiquotable). In the
case where an attribute name is just a single antiquotation, the quotes
can be dropped:
{ foo = 123; }.${bar} or 456
This will evaluate to `123` if `bar` evaluates to `"foo"` when coerced
to a string and `456` otherwise (again assuming `bar` is antiquotable).
In the special case where an attribute name inside of a set declaration
evaluates to `null` (which is normally an error, as `null` is not
antiquotable), that attribute is simply not added to the set:
{ ${if foo then "bar" else null} = true; }
This will evaluate to `{}` if `foo` evaluates to `false`.
A set that has a `__functor` attribute whose value is callable (i.e. is
itself a function or a set with a `__functor` attribute whose value is
callable) can be applied as if it were a function, with the set itself
passed in first , e.g.,
let add = { __functor = self: x: x + self.x; };
inc = add // { x = 1; };
in inc 1
evaluates to `2`. This can be used to attach metadata to a function
without the caller needing to treat it specially, or to implement a form
of object-oriented programming, for example.
1. It's parsed as an expression that selects the attribute `sh` from
the variable `builder`.

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# Building and Testing
You can now try to build Hello. Of course, you could do `nix-env -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 -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 [`-A`](#opt-attr) option selects the `hello` attribute. 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 `-o` switch to give the symlink another name.
Nix has 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 paths will not be registered as valid. If you try to build the
derivation again, Nix will remove the output paths if they exist (e.g.,
because the builder died half-way through `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 -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`).

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# A Simple Nix Expression
This section shows how to add and test the [GNU Hello
package](http://www.gnu.org/software/hello/hello.html) to the Nix
Packages collection. Hello is a program that prints out the text “Hello,
world\!”.
To add a package to the Nix Packages collection, you generally need to
do three things:
1. Write a Nix expression for the package. This is a file that
describes all the inputs involved in building the package, such as
dependencies, sources, and so on.
2. Write a *builder*. This is a shell script\[1\] that actually builds
the package from the inputs.
3. Add the package to the file `pkgs/top-level/all-packages.nix`. The
Nix expression written in the first step is a *function*; it
requires other packages in order to build it. In this step you put
it all together, i.e., you call the function with the right
arguments to build the actual package.
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1. In fact, it can be written in any language, but typically it's a
`bash` shell script.

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This chapter shows you how to write Nix expressions, which instruct Nix
how to build packages. It starts with a simple example (a Nix expression
for GNU Hello), and then moves on to a more in-depth look at the Nix
expression language.
> **Note**
>
> This chapter is mostly about the Nix expression language. For more
> extensive information on adding packages to the Nix Packages
> collection (such as functions in the standard environment and coding
> conventions), please consult [its
> manual](http://nixos.org/nixpkgs/manual/).