Install a bunch of packages. First check if they can be found in repos, then check if they are installed. If yes to first and no to second, add to an array, then install the entire array.

install_a_bunch

#!/bin/bash
function install_a_bunch() {
  PKGS_INST=( )
  for pkg in $1; do
    CAN_WE_FIND_IT=`sudo apt-get -qq --dry-run install $pkg`
    if [ ! "$CAN_WE_FIND_IT" == 100 ]; then
      IS_IT_THERE=`dpkg -l | grep $pkg`
      if [[ ! "$IS_IT_THERE" ]]; then
        PKGS_INST=( "${PKGS_INST[@]}" "${pkg}" )
      fi
    fi
  done
  if [[ ! ${#PKGS_INST[@]} == 0 ]]; then
    echo "Packages to install ${PKGS_INST[@]}"
    sudo apt-get install ${PKGS_INST[@]}
  else
    echo "Nothing to install"
  fi
}

### Add the packages you'd like below those are just there for testing and illustration.
pkgs=(nmap asdflgk)
install_a_bunch $pkgs