Copying package selections with debian/ubuntu

This is documented in lots of places but it seems to always take me a while to find it. Here is how to install the set of packages from one machine to another.

1. On the source machine: sudo dpkg --get-selections > output_file
2. Transfer the output file to the target machine
3. On the target machine: sudo dpkg --set-selections <>
4. On the target machine: sudo apt-get -u dselect-upgrade