Nix 0.8 (2005-04-11) - Lix Reference Manual

# Release 0.8 (2005-04-11)

NOTE: the hashing scheme in Nix 0.8 changed (as detailed below). As a result, `nix-pull` manifests and channels built for Nix 0.7 and below will not work anymore. However, the Nix expression language has not changed, so you can still build from source. Also, existing user environments continue to work. Nix 0.8 will automatically upgrade the database schema of previous installations when it is first run.

If you get the error message

    you have an old-style manifest `/nix/var/nix/manifests/[...]'; please
    delete it

you should delete previously downloaded manifests:

    $ rm /nix/var/nix/manifests/*

If `nix-channel` gives the error message

    manifest `http://catamaran.labs.cs.uu.nl/dist/nix/channels/[channel]/MANIFEST'
    is too old (i.e., for Nix <= 0.7)

then you should unsubscribe from the offending channel (`nix-channel --remove URL`; leave out `/MANIFEST`), and subscribe to the same URL, with `channels` replaced by `channels-v3` (e.g., http://catamaran.labs.cs.uu.nl/dist/nix/channels-v3/nixpkgs-unstable).

Nix 0.8 has the following improvements:

- The cryptographic hashes used in store paths are now 160 bits long, but encoded in base-32 so that they are still only 32 characters long (e.g., `/nix/store/csw87wag8bqlqk7ipllbwypb14xainap-atk-1.9.0`). (This is actually a 160 bit truncation of a SHA-256 hash.)

- Big cleanups and simplifications of the basic store semantics. The notion of “closure store expressions” is gone (and so is the notion of “successors”); the file system references of a store path are now just stored in the database.

  For instance, given any store path, you can query its closure:

      $ nix-store -qR $(which firefox)
      ... lots of paths ...

  Also, Nix now remembers for each store path the derivation that built it (the “deriver”):

      $ nix-store -qR $(which firefox)
      /nix/store/4b0jx7vq80l9aqcnkszxhymsf1ffa5jd-firefox-1.0.1.drv

  So to see the build-time dependencies, you can do

      $ nix-store -qR $(nix-store -qd $(which firefox))

  or, in a nicer format:

      $ nix-store -q --tree $(nix-store -qd $(which firefox))

  File system references are also stored in reverse. For instance, you can query all paths that directly or indirectly use a certain Glibc:

      $ nix-store -q --referrers-closure \
          /nix/store/8lz9yc6zgmc0vlqmn2ipcpkjlmbi51vv-glibc-2.3.4

- The concept of fixed-output derivations has been formalised. Previously, functions such as `fetchurl` in Nixpkgs used a hack (namely, explicitly specifying a store path hash) to prevent changes to, say, the URL of the file from propagating upwards through the dependency graph, causing rebuilds of everything. This can now be done cleanly by specifying the `outputHash` and `outputHashAlgo` attributes. Nix itself checks that the content of the output has the specified hash. (This is important for maintaining certain invariants necessary for future work on secure shared stores.)

- One-click installation :-) It is now possible to install any top-level component in Nixpkgs directly, through the web — see, e.g., http://catamaran.labs.cs.uu.nl/dist/nixpkgs-0.8/. All you have to do is associate `/nix/bin/nix-install-package` with the MIME type `application/nix-package` (or the extension `.nixpkg`), and clicking on a package link will cause it to be installed, with all appropriate dependencies. If you just want to install some specific application, this is easier than subscribing to a channel.

- `nix-store -r PATHS` now builds all the derivations PATHS in parallel. Previously it did them sequentially (though exploiting possible parallelism between subderivations). This is nice for build farms.

- `nix-channel` has new operations `--list` and `--remove`.

- New ways of installing components into user environments:

  - Copy from another user environment:

        $ nix-env -i --from-profile .../other-profile firefox

  - Install a store derivation directly (bypassing the Nix expression language entirely):

        $ nix-env -i /nix/store/z58v41v21xd3...-aterm-2.3.1.drv

    (This is used to implement `nix-install-package`, which is therefore immune to evolution in the Nix expression language.)

  - Install an already built store path directly:

        $ nix-env -i /nix/store/hsyj5pbn0d9i...-aterm-2.3.1

  - Install the result of a Nix expression specified as a command-line argument:

        $ nix-env -f .../i686-linux.nix -i -E 'x: x.firefoxWrapper'

    The difference with the normal installation mode is that `-E` does not use the `name` attributes of derivations. Therefore, this can be used to disambiguate multiple derivations with the same name.

- A hash of the contents of a store path is now stored in the database after a successful build. This allows you to check whether store paths have been tampered with: `nix-store --verify --check-contents`.

- Implemented a concurrent garbage collector. It is now always safe to run the garbage collector, even if other Nix operations are happening simultaneously.

  However, there can still be GC races if you use `nix-instantiate` and `nix-store --realise` directly to build things. To prevent races, use the `--add-root` flag of those commands.

- The garbage collector now finally deletes paths in the right order (i.e., topologically sorted under the “references” relation), thus making it safe to interrupt the collector without risking a store that violates the closure invariant.

- Likewise, the substitute mechanism now downloads files in the right order, thus preserving the closure invariant at all times.

- The result of `nix-build` is now registered as a root of the garbage collector. If the `./result` link is deleted, the GC root disappears automatically.

- The behaviour of the garbage collector can be changed globally by setting options in `/nix/etc/nix/nix.conf`.

  - `gc-keep-derivations` specifies whether deriver links should be followed when searching for live paths.

  - `gc-keep-outputs` specifies whether outputs of derivations should be followed when searching for live paths.

  - `env-keep-derivations` specifies whether user environments should store the paths of derivations when they are added (thus keeping the derivations alive).

- New `nix-env` query flags `--drv-path` and `--out-path`.

- `fetchurl` allows SHA-1 and SHA-256 in addition to MD5. Just specify the attribute `sha1` or `sha256` instead of `md5`.

- Manual updates.