why shell doesn't show anything after the command echo $VERSION

Question

Can somebody tell me why my command language interpreter shell shows me an empty string as the result after the command

$ echo $VERSION

And only after executing this command

$ . /etc/os-release

it begins to show me the result of that command

$ echo $VERSION

18.04.3 LTS (Bionic Beaver)

can you, please, explain me what happens after that command? Why had not shell shown it before? Why does shell show it now?

Answer 1

Running ". /etc/os-release" will run all lines in file "/etc/os-release" in your current shell; It seems that the file you are sourcing has a line like below that will set environment variable "VERSION" in your session:

VERSION="18.04.3 LTS (Bionic Beaver)"

If you are using bash then "source" will give you the same result: "source /etc/os-release"

You can check these in detail: https://ss64.com/bash/source.html or https://superuser.com/questions/46139/what-does-source-do