~/d/nixpkgs_upstream   master   sudo nixos-rebuild switch --upgrade
downloading Nix expressions from ‘https://d3g5gsiof5omrk.cloudfront.net/nixos/unstable/nixos-18.03pre121925.3eccd0b11d1/nixexprs.tar.xz’...
downloading ‘https://d3g5gsiof5omrk.cloudfront.net/nixos/unstable/nixos-18.03pre121925.3eccd0b11d1/nixexprs.tar.xz’... [0/0 KiB, 0.0 KiB/s]
unpacking channels...
building Nix...
building the system configuration...
activating the configuration...
setting up /etc...
setting up tmpfiles