Maintaining all packages when reinstalling the ubuntu OS?

[Peng Yu]

Hi,

I need to reinstall ubuntu 10.04 LTS on my machine. But I'd like to
record what ubuntu packages are installed so that I can later on
reinstall these packages easily. I'm wondering if there is a command
to keep track of all the additional packages installed. It is better
that this information is dumped into a file so that it can be read
some other command to easily reinstall all these packages. Is there
such commands for saving the packages info and reinstall the packages?

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Regards,
Peng