nix-channel

1 Nix

nix-channel

manage Nix channels

        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. See also sec-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.nixpkgsVersion'
    "14.04.527.0e935f1"

    $ nix-channel --rollback
    switching from generation 483 to 482

    $ nix-instantiate --eval -E '(import <nixpkgs> {}).lib.nixpkgsVersion'
    "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”.