With the command <command>nix-env</command>, you can install and uninstall packages from the command line. For instance, to install Mozilla Thunderbird:
If you invoke this as root, the package is installed in the Nix profile <filename>/nix/var/nix/profiles/default</filename> and visible to all users of the system; otherwise, the package ends up in <filename>/nix/var/nix/profiles/per-user/<replaceable>username</replaceable>/profile</filename> and is not visible to other users. The <option>-A</option> flag specifies the package by its attribute name; without it, the package is installed by matching against its package name (e.g. <literal>thunderbird</literal>). The latter is slower because it requires matching against all available Nix packages, and is ambiguous if there are multiple matching packages.
and then running <literal>nix-env -i</literal> again. Other packages in the profile are <emphasis>not</emphasis> affected; this is the crucial difference with the declarative style of package management, where running <command>nixos-rebuild switch</command> 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:
