Many versions of Linux already have R installed by default!
For more info on this go to this page:
https://cran.cnr.berkeley.edu/
Check out the link: Download R for linux and click on the appropriate link for your version of Linux.
To manually install R (if you don't already have it):
The current version of R available when a new version of Ubuntu is released is always available in the universe repository. To install R:
sudo apt-get install r-base r-base-dev
This will install R, the recommended packages, and files needed to install many additional packages (but not all) from within R.
Installing RStudio
if you want to have RStudio icon on the launcher (all the icons on the left hand side of the screen)
Post to the QA forums if you have any questions!