title: r install.packages to install downloaded packages
body: <p>I am trying to install and update packages. install.packages(ask=FALSE) downloads the packages but if there is an error, it aborts. If I run it again, it again downloads the packages. How can it check and install already downloaded packages on repeat running rather than downloading every time?</p>
  
  <p>I am using Debian Stable Linux with backports repository, as mentioned on : <a href="http://cran.r-project.org/bin/linux/debian/README" rel="nofollow">http://cran.r-project.org/bin/linux/debian/README</a></p>