Create a backup of what packages are currently installed

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Create a backup of what packages are currently installed:
sudo dpkg --get-selections > list.txt
Then (on another system) restore installations from that list:
sudo dpkg --clear-selections
sudo dpkg --set-selections < list.txt
To get rid of stale packages
sudo apt-get autoremove
To get installed like at backup time
sudo apt-get dselect-upgrade