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+Title: 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:
+
+```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
+
+  - `/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”.