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Channels
If you want to stay up to date with a set of packages, it’s not very
convenient to manually download the latest set of Nix expressions for
those packages and upgrade using nix-env. Fortunately, there’s a
better way: Nix channels.
A Nix channel is just a URL that points to a place that contains a set
of Nix expressions and a manifest. Using the command
nix-channel you can automatically
stay up to date with whatever is available at that URL.
To see the list of official NixOS channels, visit https://nixos.org/channels.
You can “subscribe” to a channel using nix-channel --add, e.g.,
$ nix-channel --add https://nixos.org/channels/nixpkgs-unstable
subscribes you to a channel that always contains that latest version of
the Nix Packages collection. (Subscribing really just means that the URL
is added to the file ~/.nix-channels, where it is read by subsequent
calls to nix-channel --update.) You can “unsubscribe” using nix-channel --remove:
$ nix-channel --remove nixpkgs
To obtain the latest Nix expressions available in a channel, do
$ nix-channel --update
This downloads and unpacks the Nix expressions in every channel
(downloaded from url/nixexprs.tar.bz2). It also makes the union of
each channel’s Nix expressions available by default to nix-env
operations (via the symlink ~/.nix-defexpr/channels). Consequently,
you can then say
$ nix-env -u
to upgrade all packages in your profile to the latest versions available in the subscribed channels.