Let me articulate the real problem.
As you would have guessed, just knowing the version isn't my real problem. Why I want to know the version is to check if my upgrade went through or not. And, why I want to upgrade, is because I am getting a plethora or errors on rstudio on the ubuntu server (to be clear it is non-pro rstudio) like a lot of packages not getting installed, the help not working, startup errors like these
Error in tools:::httpdPort > 0 :
comparison (6) is possible only for atomic and list types
and whenever I search for the solution the only advice I get is, upgrade your Rstudio, or upgrade your Rstudio or downgrade R version*.
@mara I request you to take a look at this SO answer as this answer may give some clues as to what is causing all this problem.
I upgraded rstudio painstakingly using this post but am not sure it has really upgraded because I still see the same problems. Perhaps there is another way to check if the upgrade went through.
Just for completion, I got following advice to check R version from an Rstudio employee here but it doesn't work either.