passing R variable to shell

You can use back-ticks (`) in most shells to capture output. So print 
the value you want using R's cat() function, and capture it thus: 

$ cat test.R 
string <- 'TEST' 
cat(string) 

$ v=`R --slave --no-save < test.R ` 
$ echo $v 
TEST 

bash shell also allows $( ) notation: 

$ v=$(R --slave --no-save < test.R ) 

 note the use of --slave to make R shut up about itself.