[lc@ashenvale ~]$ nix-env -f https://github.com/obsidiansystems/obelisk/archive/master.tar.gz -iA command
warning: Nix search path entry '/nix/var/nix/profiles/per-user/root/channels/nixpkgs' does not exist, ignoring
warning: Nix search path entry '/nix/var/nix/profiles/per-user/root/channels' does not exist, ignoring
installing 'obelisk-command-0.1'
error: while setting up the build environment: getting attributes of path '': No such file or directory