Advanced Attributes
Derivations can declare some infrequently used optional attributes.
-
allowedReferences
The optional attributeallowedReferences
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 toallowedReferences
, 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 thatfoobar
itself doesn't introduce any other dependency itself. -
disallowedReferences
The optional attributedisallowedReferences
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 todisallowedReferences
, 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 onfoobar
. -
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 bynix-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 filelibfoo-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 (acpio
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 lineimpureEnvVars = [ "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 (see below), 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 thenix-build
.If the
configurable-impure-env
experimental feature is enabled, these environment variables can also be controlled through theimpure-env
configuration setting. -
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*
andname
attributes, while all other attributes are ignored for the purpose of computing the output path. (Thename
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 likesha256sum
orsha1sum
produce).This is the default.
-
"recursive"
The hash is computed over the NAR archive dump of the output (i.e., the result ofnix-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 thenix-hash
command for information about converting to and from base-32 notation.) -
-
Warning This attribute is part of an experimental feature.
To use this attribute, you must enable the
ca-derivations
experimental feature. For example, in nix.conf you could add:extra-experimental-features = ca-derivations
If this attribute is set to
true
, then the derivation outputs will be stored in a content-addressed location rather than the traditional input-addressed one.Setting this attribute also requires setting
outputHashMode
andoutputHashAlgo
like for fixed-output derivations (see above).It also implicitly requires that the machine to build the derivation must have the
ca-derivations
system feature. -
passAsFile
A list of names of attributes that should be passed via files rather than environment variables. For example, if you havepassAsFile = ["big"]; big = "a very long string";
then when the builder runs, the environment variable
bigPath
will contain the absolute path to a temporary file containinga very long string
. That is, for any attribute x listed inpassAsFile
, Nix will pass an environment variablexPath
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 totrue
and distributed building is enabled, then, if possible, the derivation will be built locally instead of being forwarded to a remote machine. This is useful for derivations that are cheapest to build locally. -
allowSubstitutes
If this attribute is set tofalse
, then Nix will always build this derivation (locally or remotely); it will not try to substitute its outputs. This is useful for derivations that are cheaper to build than to substitute.This attribute can be ignored by setting
always-allow-substitutes
totrue
.Note
If set to
false
, thebuilder
should be able to run on the system type specified in thesystem
attribute, since the derivation cannot be substituted. -
__structuredAttrs
If the special attribute__structuredAttrs
is set totrue
, the other derivation attributes are serialised into a file in JSON format. The environment variableNIX_ATTRS_JSON_FILE
points to the exact location of that file both in a build and anix-shell
. This obviates the need forpassAsFile
since JSON files have no size restrictions, unlike process environments.It also makes it possible to tweak derivation settings in a structured way; see
outputChecks
for example.As a convenience to Bash builders, Nix writes a script that initialises shell variables corresponding to all attributes that are representable in Bash. The environment variable
NIX_ATTRS_SH_FILE
points to the exact location of the script, both in a build and anix-shell
. This includes non-nested (associative) arrays. For example, the attributehardening.format = true
ends up as the Bash associative array element${hardening[format]}
. -
outputChecks
When using structured attributes, theoutputChecks
attribute allows defining checks per-output.In addition to
allowedReferences
,allowedRequisites
,disallowedReferences
anddisallowedRequisites
, the following attributes are available:maxSize
defines the maximum size of the resulting store object.maxClosureSize
defines the maximum size of the output's closure.ignoreSelfRefs
controls whether self-references should be considered when checking for allowed references/requisites.
Example:
__structuredAttrs = true; outputChecks.out = { # The closure of 'out' must not be larger than 256 MiB. maxClosureSize = 256 * 1024 * 1024; # It must not refer to the C compiler or to the 'dev' output. disallowedRequisites = [ stdenv.cc "dev" ]; }; outputChecks.dev = { # The 'dev' output must not be larger than 128 KiB. maxSize = 128 * 1024; };
-
When using structured attributes, the attribute
unsafeDiscardReferences
is an attribute set with a boolean value for each output name. If set totrue
, it disables scanning the output for runtime dependencies.Example:
__structuredAttrs = true; unsafeDiscardReferences.out = true;
This is useful, for example, when generating self-contained filesystem images with their own embedded Nix store: hashes found inside such an image refer to the embedded store and not to the host's Nix store.
-
If a derivation has the
requiredSystemFeatures
attribute, then Nix will only build it on a machine that has the corresponding features set in itssystem-features
configuration.For example, setting
requiredSystemFeatures = [ "kvm" ];
ensures that the derivation can only be built on a machine with the
kvm
feature.