nix-env
    --dry-run
    -v, --verbose

    -u|--upgrade [*]
        upgrade (all packages if argument is not specified or just specified package[s])
        https://nixos.org/manual/nix/stable/#operation-upgrade
        # nix-channel --update nixpkgs
        --dry-run
            dry run

    -i|--install args...
        !!!
        https://nixos.wiki/wiki/FAQ#Why_not_use_nix-env_-i_hello.3F
        -f <some-stuff.nix> -E <specific-nix-expr>
        -f https://github.com/NixOS/nixpkgs-channels/archive/nixos-14.12.tar.gz -iA firefox
            A|--attr
                install attribute [nixpgks.<name>]
                ... Alternatively, like for nix-build, one can use [-A] to specify an attribute name instead of a derivation name:
                $ nix-env -f '<mypkgs>' -i -A graphviz

    -e|--uninstall drvnames...
        https://nixos.org/manual/nix/stable/#operation-uninstall
        -e '*'
            for uninstall everyting from the user environment

    --list-generations
    -G, --switch-generation <num>
        https://nixos.org/manual/nix/stable/#operation-switch-generation
    --delete-generations generations...
        https://nixos.org/manual/nix/stable/#operation-delete-generations
    --rollback
        https://nixos.org/manual/nix/stable/#operation-rollback

    -q|--query [--installed]
        list installed derivations
    -qa [package]
        search for a package on a channel
    -qaf /path/to/nixpkgs
        ... at the custom path
    -qas
        status of available packages
        I - installed in your current user env
        P - present on your system
        S - substitute for the package (prebuilt package)
            https://cache.nixos.org - cache of binary packages
    -qaP ...
        nixos.<bla-bla> -> pkgs.<bla-bla> at configuration.nix
        ????
        ? list stuff
   
        

    -f <dir>
        In order to specify an alternative to ~/.nix-defexpr it's possible to use the [-f] option: