finding out package and versions in apt

search available packages with keyword

$ apt-cache search <keyword>

list available versions of a package. For example, if I want to list all available versions for package libcudnn7

$ apt list -a libcudnn7

Before running this, running apt update is recommended.

After finding a specific version that I want to install, then install the packge with a specific version like this

$ apt install libcudnn7=7.6.5.32-1+cuda10.1