How can I check if a package is installed and install it if not?

I'm working on a Ubuntu system and currently this is what I'm doing:

if ! which command > /dev/null; then
   echo -e "Command not found! Install? (y/n) \c"
   read
   if "$REPLY" = "y"; then
      sudo apt-get install command
   fi
fi

Is this what most people would do? Or is there a more elegant solution?

To check if packagename was installed, type:

dpkg -s <packagename>

You can also use dpkg-query that has a neater output for your purpose, and accepts wild cards, too.

dpkg-query -l <packagename>

To find what package owns the command, try:

dpkg -S `which <command>`

You can use these in an if statement in bash like this:

if dpkg -s desired-pkg-name &>/dev/null; then
  echo 'desired-pkg-name is installed'
fi

For further details, see article Find out if package is installed in Linux and dpkg cheat sheet.