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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`]{#attr-system} whose value must be a
string specifying a Nix system type, such as `"i686-linux"` or
`"x86_64-darwin"`. (To figure out your system type, run `nix -vv
--version`.) The build can only be performed on a machine and
operating system matching the system type. (Nix can automatically
[forward builds for other
platforms](../advanced-topics/distributed-builds.md) by forwarding
them to other machines.)
- 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:
```nix
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
```bash
./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.
```nix
buildInputs = [ pkg.lib pkg.headers ];
```
The first element of `outputs` determines the *default output*.
Thus, you could also write
```nix
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.
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