apt

If I need to reinstall a Ubuntu/Debian-based Linux OS (or mirror a software selection to another machine), this is how it can be done. On the source machine:

su
dpkg --get-selections > ~/Package.list
cp -R /etc/apt/sources.list* ~/
apt-key exportall > ~/repository.keys


Then just copy the files to the target machine:

Search for packages containing the word emacs in their name or description:
apt-cache search emacs
Install package emacs21:
apt-get install emacs21
Remove package wine including all configuration files:
apt-get remove --purge wine
Reinstall a package:
apt-get --reinstall install wine
Sometimes, you need to use dpkg to reconfigure a package, e.g. here the bittorrent sync package:
dpkg-reconfigure btsync