One of the easiest things to forget about your Linux system is the information regarding the installed kernel. I remember that back when the 64 bits system where introduced, I had a mix of various Linux systems running in virtual machines and at times I couldn’t remember where I was using the back then usual 32 bits version and in which I had installed the new 64 bits one.
Here we have a command line instruction that reveals all kind of information about the installed kernel:
That in this Debian system, for example, returns this:
Linux dhcppc0 4.9.0-8-amd64 #1 SMP Debian 4.9.130-2 (2018-10-27) x86_64 GNU/Linux
Form the output above we can see the system is running with the 4.9 version of the Linux kernel.