Execution halted - R command line parameters

Question

I have a R script and I would like to pass some filenames/parameters to my Rscript and to execute it from a terminal. I used this command:

fileA='aaaa.txt'
fileB='bbbb.txt'
n=100
string='hello'
R CMD BATCH --vanilla '--args $fileA $fileB $n $string' Rscript_file.R

I added this line in my Rscript_file.R file:

args <- commandArgs(trailingonly=TRUE)

but I got this error:

> args <- commandArgs(trailingonly=TRUE)
Error in commandArgs(trailingonly = TRUE) : 
  unused argument (trailingonly = TRUE)
Execution halted

Any ideas? thanks