[R]Reset the environment variable for R in El Capitan

The issue is I can't call R in terminal after El Capitan has been installed. This is the result of the newly banned writing permission to usr/bin. See this post: R in El Capitan public beta (new security model).

1. Find the R binary.
I have the Rstudio, where I can open R console. In Rstudio, use the following command to track where R binary is or RHOME.
If you can't track your directory, you can also look for R in this path. My system is El Capitan.

2. Modify the path profile.
Add the directory to your system path. In terminal:
$ sudo nano /etc/paths
Enter your password, when prompted.
Go to the bottom of the file, and enter the path you found in the first step.
Hit control-x to quit.
Enter “Y” to save the modified buffer.
Hit enter t confirm when prompted by "File name to write".

Major reference:
Add to the PATH on Mac OS X 10.8 Mountain Lion
Set environment variables on Mac OS X Lion