nix-channel

nix-channel {--add url [name] | --remove name | --list | --update [names…] | --rollback [generation] }

A Nix channel is a mechanism that allows you to automatically stay up-to-date with a set of pre-built Nix expressions. A Nix channel is just a URL that points to a place containing a set of Nix expressions.

To see the list of official NixOS channels, visit https://nixos.org/channels.

This command has the following operations:

• --add url [name]
  Adds a channel named name with URL url to the list of subscribed channels. If name is omitted, it defaults to the last component of url, with the suffixes -stable or -unstable removed.
• --remove name
  Removes the channel named name from the list of subscribed channels.
• --list
  Prints the names and URLs of all subscribed channels on standard output.
• --update [names…]
  Downloads the Nix expressions of all subscribed channels (or only those included in names if specified) and makes them the default for nix-env operations (by symlinking them from the directory ~/.nix-defexpr).
• --rollback [generation]
  Reverts the previous call to nix-channel --update. Optionally, you can specify a specific channel generation number to restore.

Note that --add does not automatically perform an update.

The list of subscribed channels is stored in ~/.nix-channels.

To subscribe to the Nixpkgs channel and install the GNU Hello package:

$ nix-channel --add https://nixos.org/channels/nixpkgs-unstable
$ nix-channel --update
$ nix-env -iA nixpkgs.hello

You can revert channel updates using --rollback:

$ nix-instantiate --eval -E '(import <nixpkgs> {}).lib.version'
"14.04.527.0e935f1"
$ nix-channel --rollback
switching from generation 483 to 482
$ nix-instantiate --eval -E '(import <nixpkgs> {}).lib.version'
"14.04.526.dbadfad"

• /nix/var/nix/profiles/per-user/username/channels
  nix-channel uses a nix-env profile to keep track of previous versions of the subscribed channels. Every time you run nix-channel --update, a new channel generation (that is, a symlink to the channel Nix expressions in the Nix store) is created. This enables nix-channel --rollback to revert to previous versions.
• ~/.nix-defexpr/channels
  This is a symlink to /nix/var/nix/profiles/per-user/username/channels. It ensures that nix-env can find your channels. In a multi-user installation, you may also have ~/.nix-defexpr/channels_root, which links to the channels of the root user.

A channel URL should point to a directory containing the following files:

• nixexprs.tar.xz
  A tarball containing Nix expressions and files referenced by them (such as build scripts and patches). At the top level, the tarball should contain a single directory. That directory must contain a file default.nix that serves as the channel’s “entry point”.