nix-super/doc/manual/src/language/derivations.md

Derivations

The most important built-in function is derivation, which is used to describe a single derivation: a build task to run a process on precisely defined input files and 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 build. It outputs an attribute set, and produces a store derivation as a side effect of evaluation.

Input attributes

Required

• [name]{#attr-name} (String)

  Symbolic name for the derivation. It is appended to the store paths of the resulting store derivation and its output paths.

  Example: name = "hello";

• [system]{#attr-system} (String)

  The system type on which the builder executable can be run.

  Nix will only build derivations where the system attribute matches the current system configuration option. It can automatically build on other platforms by forwarding build requests to other machines.

  Examples:

  system = "x86_64-linux";

  system = builtins.currentSystem;

  builtins.currentSystem has the value of the system configuration option, and defaults to the system type of the current Nix installation.

• [builder]{#attr-builder} (Path | String)

  Path to an executable that will perform the build.

  Examples:

  builder = "/bin/bash";

  builder = ./builder.sh;

  builder = "${pkgs.python}/bin/python";

Optional

• [args]{#attr-args} (List of String) Default: [ ]

  Command-line arguments to be passed to the builder executable.

  Example: args = [ "-c" "echo hello world > $out" ];

• [outputs]{#attr-outputs} (List of String) Default: [ "out" ]

  Symbolic outputs of the derivation. Each output name is passed to the builder executable as an environment variable with its value set to the corresponding output path.

  By default, a derivation produces a single output path called out. However, derivations can produce multiple output paths. This allows the associated store objects and their closures to be copied or garbage-collected separately.

  Examples:

  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.

• 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 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.

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_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.