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Backing up and restoring Debian 
packages
As you maintain your server, your list of installed packages will grow. If, for some 
reason, you needed to rebuild your server, you would need to reproduce exactly 
what you had installed before, which can be a pain. It's always recommended that 
you document all changes made to your server via a change control process, but, 
at the very least, keeping track of which packages are installed is an absolute must. 
In some cases, a server may only include one or two extra packages in order to 
meet its goal, but, in other cases, you may need an exact combination of software 
and libraries in order to get things working like they were. Thankfully, the 
dpkg
 
command allows us to export and import a list of packages to install.
To export a list of installed packages, we can use the following command:
dpkg --get-selections > packages.list
This command will dump a list of package selections to a standard text file. If you 
open it, you'll see a list of your installed packages, one per line. A typical line within 
the exported file will look similar to the following:
tmux install

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Managing Software Packages