This works! Do I need to do this command sequences every time after nixos-rebuild switch?
[root@earth:~]# nix-channel --add https://nixos.org/channels/nixos-unstable nixos
[root@earth:~]# nix-channel --list
nixos https://nixos.org/channels/nixos-unstable
[root@earth:~]# nix-channel --update
unpacking channels...
[root@earth:~]# command-not-found hello
The program ‘hello’ is currently not installed. It is provided by
several packages. You can install it by typing one of the following:
nix-env -iA nixos.hello
nix-env -iA nixos.mbedtls
nix-env -iA nixos.perkeep