A character scalar giving a colon (Unix) or semicolon (Windows)
separated list of directories in which to look for the program.
The default value is
Sys.getenv("PATH").
PATHEXT
On Windows, a semicolon-separated list of file name extensions
which are appended to executable files. By default this is the
value of
Sys.getenv("PATH"), or, if that is not
set to anything ".EXE;.BAT".
VALUE:
A character vector with names. Each element gives the full
path to the executable given by its name.