A character string giving the R home directory or path to a
  particular component.  Normally the components are all subdirectories
  of the R home directory, but this need not be the case in a Unix-like
  installation.
The value for "modules" and on Windows "bin" is a
  sub-architecture-specific location.
On a Unix-alike, the constructed paths are based on the current
  values of the environment variables R_HOME and where set
  R_SHARE_DIR, R_DOC_DIR and R_INCLUDE_DIR (these are
  set on startup and should not be altered).
On Windows the values of R.home() and R_HOME are
  switched to the 8.3 short form of path elements if required and if
  the Windows service to do that is enabled.  The value of
  R_HOME is set to use forward slashes (since many package
  maintainers pass it unquoted to shells, for example in
  Makefiles).