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The R home directory

The R home directory contains the configuration and installation files. It is identified with the R_HOME environment variable.

Determine the path of the R home directory

In an R session, the value of R_HOME can be displayed like so: 
> Sys.getenv('R_HOME')
[1] "/usr/lib/R"
Alternatively, it can be shown on a shell by executing: 
$ R RHOME
/usr/lib/ R

Files found in the R home directory

etc/Renviron

R_HOME/etc/Renviron contains environment variables to configure some aspects of R.
This file is read early in the start-up processing.
In my current Debian installation (apt install r-base), /usr/lib/R/etc/Renviron is a symlink to /etc/R/Renviron.

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