Derivations
The most important built-in function is derivation, which is used to describe a single derivation:
a specification for running an executable on precisely defined input files to repeatably produce output files at uniquely determined file system paths.
It takes as input an attribute set, the attributes of which specify the inputs to the process. It outputs an attribute set, and produces a store derivation as a side effect of evaluation.
Input attributes
Required
-
A symbolic name for the derivation. It is added to the store path of the corresponding store derivation as well as to its output paths.
Example
derivation { name = "hello"; # ... }The store derivation's path will be
/nix/store/<hash>-hello.drv. The output paths will be of the form/nix/store/<hash>-hello[-<output>] -
The system type on which the
builderexecutable is meant to be run.A necessary condition for Nix to build derivations locally is that the
systemattribute matches the currentsystemconfiguration option. It can automatically build on other platforms by forwarding build requests to other machines.Example
Declare a derivation to be built on a specific system type:
derivation { # ... system = "x86_64-linux"; # ... }Example
Declare a derivation to be built on the system type that evaluates the expression:
derivation { # ... system = builtins.currentSystem; # ... }builtins.currentSystemhas the value of thesystemconfiguration option, and defaults to the system type of the current Nix installation. -
Path to an executable that will perform the build.
Example
Use the file located at
/bin/bashas the builder executable:derivation { # ... builder = "/bin/bash"; # ... };Example
Copy a local file to the Nix store for use as the builder executable:
derivation { # ... builder = ./builder.sh; # ... };Example
Use a file from another derivation as the builder executable:
let pkgs = import <nixpkgs> {}; in derivation { # ... builder = "${pkgs.python}/bin/python"; # ... };
Optional
-
Default:
[ ]Command-line arguments to be passed to the
builderexecutable.Example
Pass arguments to Bash to interpret a shell command:
derivation { # ... builder = "/bin/bash"; args = [ "-c" "echo hello world > $out" ]; # ... }; -
Default:
[ "out" ]Symbolic outputs of the derivation. Each output name is passed to the
builderexecutable as an environment variable with its value set to the corresponding store path.By default, a derivation produces a single output called
out. However, derivations can produce multiple outputs. This allows the associated store objects and their closures to be copied or garbage-collected separately.Example
Imagine a library package that provides a dynamic library, header files, and documentation. A program that links against such a library doesn’t need the header files and documentation at runtime, and it doesn’t need the documentation at build time. Thus, the library package could specify:
derivation { # ... outputs = [ "lib" "dev" "doc" ]; # ... }This will cause Nix to pass environment variables
lib,dev, anddocto the builder containing the intended store paths of each output. The builder would typically do something like./configure \ --libdir=$lib/lib \ --includedir=$dev/include \ --docdir=$doc/share/docfor an Autoconf-style package.
The name of an output is combined with the name of the derivation to create the name part of the output's store path, unless it is
out, in which case just the name of the derivation is used.Example
derivation { name = "example"; outputs = [ "lib" "dev" "doc" "out" ]; # ... }The store derivation path will be
/nix/store/<hash>-example.drv. The output paths will be/nix/store/<hash>-example-lib/nix/store/<hash>-example-dev/nix/store/<hash>-example-doc/nix/store/<hash>-example
You can refer to each output of a derivation by selecting it as an attribute. The first element of
outputsdetermines the default output and ends up at the top-level.Example
Select an output by attribute name:
let myPackage = derivation { name = "example"; outputs = [ "lib" "dev" "doc" "out" ]; # ... }; in myPackage.devSince
libis the first output,myPackageis equivalent tomyPackage.lib. -
See Advanced Attributes for more, infrequently used, optional attributes.
-
Every other attribute is passed as an environment variable to the builder. Attribute values are translated to environment variables as follows:
-
Strings are passed unchanged.
-
Integral numbers are converted to decimal notation.
-
Floating point numbers are converted to simple decimal or scientific notation with a preset precision.
-
A path (e.g.,
../foo/sources.tar) causes the referenced file to be copied to the store; its location in the store is put in the environment variable. The idea is that all sources should reside in the Nix store, since all inputs to a derivation should reside in the Nix store. -
A derivation causes that derivation to be built prior to the present derivation. The environment variable is set to the store path of the derivation's default output.
-
Lists of the previous types are also allowed. They are simply concatenated, separated by spaces.
-
trueis passed as the string1,falseandnullare passed as an empty string.
-
Builder execution
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_TOPcontains the path of the temporary directory for this build. -
Also,
TMPDIR,TEMPDIR,TMP,TEMPare 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. -
PATHis set to/path-not-setto prevent shells from initialising it to their built-in default value. -
HOMEis set to/homeless-shelterto prevent programs from using/etc/passwdor the like to find the user's home directory, which could cause impurity. Usually, whenHOMEis set, it is used as the location of the home directory, even if it points to a non-existent path. -
NIX_STOREis set to the path of the top-level Nix store directory (typically,/nix/store). -
NIX_ATTRS_JSON_FILE&NIX_ATTRS_SH_FILEif__structuredAttrsis set totruefor the derivation. A detailed explanation of this behavior can be found in the section about structured attrs. -
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, thenameattribute and the output name. (The output name is omitted if it’sout.)
-
-
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
-Koption 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
setuidandsetgidbits 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.