nix-channel

# Name

`nix-channel` - manage Nix channels

# Synopsis

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

# Description

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`.

# Examples

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"

# Files

- `/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.

# Channel format

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”.