upgrade only if installed

[Kamaraju Kusumanchi <raju.mailinglists@gmail.com>]

I would like to upgrade a package only if it is installed. If the package is 
not already installed, I would not want it to be installed. Is this possible? 

Currently when I use

apt-get install <packagename>

apt-get does not distinguish between whether the package has already been 
installed or not. Is there any other program which distinguishes between the 
two?

thanks
raju

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