2/25/20
Debian/Ubuntu software is modular, and when you tell the software manager to install a piece of software, it refers to the installation's internal software index known as the local package index [1] to determine which modules/packages are required, then downloads them from a secure site, screens them for corruption by comparing them to reference data in the local package index, and places them in /var/cache/apt/archives before the actual installation process begins.
Normally, the downloaded packages are automatically deleted after installation. However, some people might want to retain the downloaded packages after installing them, such as to save copies to use in case they have to re-create the installation in the near future, such as to create a new installation to replace a botched one. Packages can become outdated quickly, and new versions might be required if you wait too long to re-create the installation, because every time a new installation is created, a new local package index must be installed if you want to install any additional software, and the new index would specify the latest versions for everything.
According to Disable auto clean in apt (https://unix.stackexchange.com/questions/499035/disable-auto-clean-in-apt ), the downloaded packages can be retained by entering the following command:
"echo 'Binary::apt::APT::Keep-Downloaded-Packages "1";' | sudo tee /etc/apt/apt.conf.d/10apt-keep-downloads" (Copy the command without the end-quotes and paste it into the terminal with Ctrl-Shift-V. In case there have been any revisions, get the command directly from the source.)
This command (not necessarily in the following order) creates a text-file named 10apt-keep-downloads, inserts the line "Binary::apt::APT::Keep-Downloaded-Packages "1";" in it, and places the file in the /etc/apt/apt.conf.d/ directory, which contains Apt configuration files, which are executed in alphanumeric order. In the past, they were lumped into a single configuration file, but it became too unwieldy.
I took a different approach, by opening the installation's text-editor with superuser privileges (by entering "sudo <text-editor name>" and then the password) and added the aforementioned line to the /etc/apt/apt.conf.d/20archive file because it pertains to the archives directory. This caused the packages to be retained, but it apparently also caused some problems. So, to be on the safe side, and for convenience, I recommend using the aforementioned command.
Notes
[1] The local package index, which is an extracted version of user-selected sections of the online package index (which is revised daily) is massive and contained in var/lib/apt/lists (the package index is also known as "package lists" when extracted). The reference data for a particular package can be found by entering "apt-cache show <package name>" (assuming that the local package index includes the section which pertains to the package of interest). For an introduction to the package index from a PC-user's perspective, see the package-index sections in my article on Apt-offline, because although it's not difficult to understand, it has to be explained in a precise manner when explaining it in writing, and I don't want to repeat it here.
