Hi Apple changed the security model in MacOSX 10.11 El Capitan. Although 10.11 is still public beta it is unlikely that the security model will be any different in the final release.
What this means is that for example the directory /usr/bin is no longer accessible (not even with root) so any software paths that link to this directory are not working any more. As for example MacTex or R.
MacTex noticed this problem and state one their website:
MacTeX installs the link /usr/texbin, pointing indirectly to the binaries of TeX Live. In El Capitan, users cannot write to the location /usr, even if they have Administrator permission. Consequently, MacTeX-2015 and BasicTeX-2015 install two symbolic links on all systems earlier than El Capitan: /usr/texbin and /Library/TeX/texbin. Only the second link is installed on future systems.
MacTeX installs four GUI apps: BibDesk, LaTeXiT, TeX Live Utility, and TeXShop. Many users have other GUI front ends and utilities. All of these programs currently are configured to find binaries in /usr/texbin. Over the summer, users should reconfigure their GUI apps to use /Library/TeX/texbin. They will notice no change. This reconfiguration will be mandatory once El Capitan is released.
Now I wonder how we can change the $PATH variable for R to work in the terminal (bash) again on OSX 10.11. ? I could not find any help on how to do that. At this moment by typing R in the terminal the following error message is produced although R installed successfully: R: command not found