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# Name
`nix-channel` - manage Nix channels
# Synopsis
`nix-channel` {`--add` url [*name*] | `--remove` *name* | `--list` | `--update` [*names…*] | `--list-generations` | `--rollback` [*generation*] }
# Description
Channels are a mechanism for referencing remote Nix expressions and conveniently retrieving their latest version.
The moving parts of channels are:
- The official channels listed at <https://nixos.org/channels>
- The user-specific list of [subscribed channels](#subscribed-channels)
- The [downloaded channel contents](#channels)
- The [Nix expression search path](@docroot@/command-ref/conf-file.md#conf-nix-path), set with the [`-I` option](#opt-i) or the [`NIX_PATH` environment variable](#env-NIX_PATH)
> **Note**
>
> The state of a subscribed channel is external to the Nix expressions relying on it.
> This may limit reproducibility.
>
> Dependencies on other Nix expressions can be declared explicitly with:
> - [`fetchurl`](@docroot@/language/builtins.md#builtins-fetchurl), [`fetchTarball`](@docroot@/language/builtins.md#builtins-fetchTarball), or [`fetchGit`](@docroot@/language/builtins.md#builtins-fetchGit) in Nix expressions
> - the [`-I` option](@docroot@/command-ref/opt-common.md#opt-I) in command line invocations
This command has the following operations:
- `--add` *url* \[*name*\]\
Add a channel *name* located at *url* to the list of subscribed channels.
If *name* is omitted, default to the last component of *url*, with the suffixes `-stable` or `-unstable` removed.
> **Note**
>
> `--add` does not automatically perform an update.
> Use `--update` explicitly.
A channel URL must point to a directory containing a file `nixexprs.tar.gz`.
At the top level, that tarball must contain a single directory with a `default.nix` file that serves as the channel’s entry point.
- `--remove` *name*\
Remove the channel *name* from the list of subscribed channels.
- `--list`\
Print the names and URLs of all subscribed channels on standard output.
- `--update` \[*names*…\]\
Download the Nix expressions of subscribed channels and create a new generation.
Update all channels if none is specified, and only those included in *names* otherwise.
- `--list-generations`\
Prints a list of all the current existing generations for the
channel profile.
Works the same way as
```
nix-env --profile /nix/var/nix/profiles/per-user/$USER/channels --list-generations
```
- `--rollback` \[*generation*\]\
Revert channels to the state before the last call to `nix-channel --update`.
Optionally, you can specify a specific channel *generation* number to restore.
{{#include ./opt-common.md}}
{{#include ./env-common.md}}
# Files
`nix-channel` operates on the following files.
{{#include ./files/channels.md}}
# Examples
Subscribe to the Nixpkgs channel and run `hello` from the GNU Hello package:
```console
$ nix-channel --add https://nixos.org/channels/nixpkgs-unstable
$ nix-channel --list
nixpkgs https://nixos.org/channels/nixpkgs
$ nix-channel --update
$ nix-shell -p hello --run hello
hello
```
Revert channel updates using `--rollback`:
```console
$ nix-instantiate --eval '<nixpkgs>' --attr lib.version
"22.11pre296212.530a53dcbc9"
$ nix-channel --rollback
switching from generation 483 to 482
$ nix-instantiate --eval '<nixpkgs>' --attr lib.version
"22.11pre281526.d0419badfad"
```
Remove a channel:
```console
$ nix-channel --remove nixpkgs
$ nix-channel --list
```
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