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# 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:
```console
$ nix-channel --add https://nixos.org/channels/nixpkgs-unstable
$ nix-channel --update
$ nix-env -iA nixpkgs.hello
```
You can revert channel updates using `--rollback`:
```console
$ 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
- `${XDG_STATE_HOME-$HOME/.local/state}/nix/profiles/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
`${XDG_STATE_HOME-$HOME/.local/state}/nix/profiles/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”.
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