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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.