With the command nix-env, you can install and uninstall packages from the command line. For instance, to install Mozilla Thunderbird: $ nix-env -iA nixos.thunderbird If you invoke this as root, the package is installed in the Nix profile /nix/var/nix/profiles/default and visible to all users of the system; otherwise, the package ends up in /nix/var/nix/profiles/per-user/username/profile and is not visible to other users. The -A flag specifies the package by its attribute name; without it, the package is installed by matching against its package name (e.g. thunderbird). The latter is slower because it requires matching against all available Nix packages, and is ambiguous if there are multiple matching packages. Packages come from the NixOS channel. You typically upgrade a package by updating to the latest version of the NixOS channel: $ nix-channel --update nixos and then running nix-env -i again. Other packages in the profile are not affected; this is the crucial difference with the declarative style of package management, where running nixos-rebuild switch causes all packages to be updated to their current versions in the NixOS channel. You can however upgrade all packages for which there is a newer version by doing: $ nix-env -u '*' A package can be uninstalled using the -e flag: $ nix-env -e thunderbird Finally, you can roll back an undesirable nix-env action: $ nix-env --rollback nix-env has many more flags. For details, see the nix-env 1 manpage or the Nix manual.