\"i686-linux\"` (note that `--argstr` prevents annoying quoting
    around shell arguments).

  - `nix-store` has a new operation `--read-log` (`-l`) `paths` that
    shows the build log of the given paths.

  - Nix now uses Berkeley DB 4.5. The database is upgraded
    automatically, but you should be careful not to use old versions of
    Nix that still use Berkeley DB 4.4.

  - The option `--max-silent-time` (corresponding to the configuration
    setting `build-max-silent-time`) allows you to set a timeout on
    builds — if a build produces no output on `stdout` or `stderr` for
    the given number of seconds, it is terminated. This is useful for
    recovering automatically from builds that are stuck in an infinite
    loop.

  - `nix-channel`: each subscribed channel is its own attribute in the
    top-level expression generated for the channel. This allows
    disambiguation (e.g. `nix-env
            -i -A nixpkgs_unstable.firefox`).

  - The substitutes table has been removed from the database. This makes
    operations such as `nix-pull` and `nix-channel --update` much, much
    faster.

  - `nix-pull` now supports bzip2-compressed manifests. This speeds up
    channels.

  - `nix-prefetch-url` now has a limited form of caching. This is used
    by `nix-channel` to prevent unnecessary downloads when the channel
    hasn’t changed.

  - `nix-prefetch-url` now by default computes the SHA-256 hash of the
    file instead of the MD5 hash. In calls to `fetchurl` you should pass
    the `sha256` attribute instead of `md5`. You can pass either a
    hexadecimal or a base-32 encoding of the hash.

  - Nix can now perform builds in an automatically generated “chroot”.
    This prevents a builder from accessing stuff outside of the Nix
    store, and thus helps ensure purity. This is an experimental
    feature.

  - The new command `nix-store
            --optimise` reduces Nix store disk space usage by finding identical
    files in the store and hard-linking them to each other. It typically
    reduces the size of the store by something like 25-35%.

  - `~/.nix-defexpr` can now be a directory, in which case the Nix
    expressions in that directory are combined into an attribute set,
    with the file names used as the names of the attributes. The command
    `nix-env
            --import` (which set the `~/.nix-defexpr` symlink) is removed.

  - Derivations can specify the new special attribute
    `allowedReferences` to enforce that the references in the output of
    a derivation are a subset of a declared set of paths. For example,
    if `allowedReferences` is an empty list, then the output must not
    have any references. This is used in NixOS to check that generated
    files such as initial ramdisks for booting Linux don’t have any
    dependencies.

  - The new attribute `exportReferencesGraph` allows builders access to
    the references graph of their inputs. This is used in NixOS for
    tasks such as generating ISO-9660 images that contain a Nix store
    populated with the closure of certain paths.

  - Fixed-output derivations (like `fetchurl`) can define the attribute
    `impureEnvVars` to allow external environment variables to be passed
    to builders. This is used in Nixpkgs to support proxy configuration,
    among other things.

  - Several new built-in functions: `builtins.attrNames`,
    `builtins.filterSource`, `builtins.isAttrs`, `builtins.isFunction`,
    `builtins.listToAttrs`, `builtins.stringLength`, `builtins.sub`,
    `builtins.substring`, `throw`, `builtins.trace`,
    `builtins.readFile`.