The best way to keep your NixOS installation up to date is to use one of the NixOS channels. A channel is a Nix mechanism for distributing Nix expressions and associated binaries. The NixOS channels are updated automatically from NixOS’s Git repository after certain tests have passed and all packages have been built. These channels are: Stable channels, such as nixos-14.12. These only get conservative bug fixes and package upgrades. For instance, a channel update may cause the Linux kernel on your system to be upgraded from 3.4.66 to 3.4.67 (a minor bug fix), but not from 3.4.x to 3.11.x (a major change that has the potential to break things). Stable channels are generally maintained until the next stable branch is created. The unstable channel, nixos-unstable. This corresponds to NixOS’s main development branch, and may thus see radical changes between channel updates. It’s not recommended for production systems. Small channels, such as nixos-14.12-small or nixos-unstable-small. These are identical to the stable and unstable channels described above, except that they contain fewer binary packages. This means they get updated faster than the regular channels (for instance, when a critical security patch is committed to NixOS’s source tree), but may require more packages to be built from source than usual. They’re mostly intended for server environments and as such contain few GUI applications. To see what channels are available, go to . (Note that the URIs of the various channels redirect to a directory that contains the channel’s latest version and includes ISO images and VirtualBox appliances.) When you first install NixOS, you’re automatically subscribed to the NixOS channel that corresponds to your installation source. For instance, if you installed from a 14.12 ISO, you will be subscribed to the nixos-14.12 channel. To see which NixOS channel you’re subscribed to, run the following as root: # nix-channel --list | grep nixos nixos https://nixos.org/channels/nixos-unstable To switch to a different NixOS channel, do # nix-channel --add https://nixos.org/channels/channel-name nixos (Be sure to include the nixos parameter at the end.) For instance, to use the NixOS 14.12 stable channel: # nix-channel --add https://nixos.org/channels/nixos-14.12 nixos If you have a server, you may want to use the “small” channel instead: # nix-channel --add https://nixos.org/channels/nixos-14.12-small nixos And if you want to live on the bleeding edge: # nix-channel --add https://nixos.org/channels/nixos-unstable nixos You can then upgrade NixOS to the latest version in your chosen channel by running # nixos-rebuild switch --upgrade which is equivalent to the more verbose nix-channel --update nixos; nixos-rebuild switch. It is generally safe to switch back and forth between channels. The only exception is that a newer NixOS may also have a newer Nix version, which may involve an upgrade of Nix’s database schema. This cannot be undone easily, so in that case you will not be able to go back to your original channel.

You can keep a NixOS system up-to-date automatically by adding the following to configuration.nix: system.autoUpgrade.enable = true; This enables a periodically executed systemd service named nixos-upgrade.service. It runs nixos-rebuild switch --upgrade to upgrade NixOS to the latest version in the current channel. (To see when the service runs, see systemctl list-timers.) You can also specify a channel explicitly, e.g. system.autoUpgrade.channel = https://nixos.org/channels/nixos-15.09;