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During development on a linux system, you probably install many packages using your favorite package manager. When you have to use a new system, or reimage your current one, it can be a pain to remember all the packages you had setup. One solution is to keep a list of the packages installed after the OS load, and then periodically generate a list of what has been added since.

On a freshly installed system, create the starting baseline list of packages:

# On Debian based systems(Ubuntu):
dpkg --get-selections > packages.txt
 
# Or CentOS/Fedora:
yum list installed > packages.txt

You can run the command again at a later time, concatenating the output into a different file so you can view what has changed since the original system setup. Use a diff tool like diff3, vimdiff, or meld:

meld packages-old.txt packages.txt

On Debian systems, once you have that file you can use it in a new or different system to mark packages to install using the –set-selection parameter:

dpkg --set-selections < packages.txt
sudo apt-get upgrade