Run R silently from command line, export results to JSON - json

Question

How might I call an R script from the shell (e.g. from Node.js exec) and export results as JSON (e.g. back to Node.js)?
The R code below basically works. It reads data, fits a model, converts the parameter estimates to JSON, and prints them to stdout:
#!/usr/bin/Rscript --quiet --slave
install.packages("cut", repos="http://cran.rstudio.com/");
install.packages("Hmisc", repos="http://cran.rstudio.com/");
install.packages("rjson", repos="http://cran.rstudio.com/");
library(rjson)
library(reshape2);
data = read.csv("/data/records.csv", header = TRUE, sep=",");
mylogit <- glm( y ~ x1 + x2 + x3, data=data, family="binomial");
params <- melt(mylogit$coefficients);
json <- toJSON(params);
json
Here's how I'd like to call it from Node...
var exec = require('child_process').exec;
exec('./model.R', function(err, stdout, stderr) {
var params = JSON.parse(stdout); // FAIL! Too much junk in stdout
});
Except the R process won't stop printing to stdout. I've tried --quiet --slave --silent which all help a little but not enough. Here's what's sent to stdout:
The downloaded binary packages are in
/var/folders/tq/frvmq0kx4m1gydw26pcxgm7w0000gn/T//Rtmpyk7GmN/downloaded_packages
The downloaded binary packages are in
/var/folders/tq/frvmq0kx4m1gydw26pcxgm7w0000gn/T//Rtmpyk7GmN/downloaded_packages
[1] "{\"value\":[4.04458733165933,0.253895751245782,-0.1142272181932,0.153106007464742,-0.00289013062471735,-0.00282580664375527,0.0970325223603164,-0.0906967639834928,0.117150317941983,0.046131890754108,6.48538603593323e-06,6.70646151749708e-06,-0.221173770066275,-0.232262366060079,0.163331098409235]}"
What's the best way to use R scripts on the command line?
Running R --silent --slave CMD BATCH model.R per the post below still results in a lot of extraneous text printed to model.Rout:
Run R script from command line

Answer 1

Those options only stop R's own system messages from printing, they won't stop another R function doing some printing. Otherwise you'll stop your last line from printing and you won't get your json to stdout!
Those messages are coming from install.packages, so try:
install.packages(-whatever-, quiet=TRUE)
which claims to reduce the amount of output. If it reduces it to zero, job done.
If not, then you can redirect stdout with sink, or run things inside capture.output.